Use ReadOnlyVariables and ReadWriteVariables properties in Scripts

For those of you just starting out with SQL Server Integration Services, you'll find a LOT of information out there on the internet.  This series (posts tagged with "Best Practice") is intended to bring the focus back to fundamentals.  I've found that it's easy to get confused with advanced techniques that seem to be targeted at your issue - but are overkill and meant for specific tricky situations.  I've done it on both sides - got suckered in to using something more complicated than I needed to, as well as written about more advanced things that got used where they didn't need to be.
The Misconception
One of those techniques is getting at SSIS variables from within a Script Task or Script component.  You'll find a lot of posts out there about the LockOneForRead, LockOneForWrite, LockForRead, LockForWrite and GetVariables methods.  Here's a small sample of posts - but go ahead and Bing/Google yourself:

• SSIS: Reading and Writing to Variables in Script Task - Arie Jones (blog)
• SSIS: Writing to a variable from a script task - Jamie Thomson (blog|twitter)
• SSIS - Variable lists for the script task are case sensitive - Simon Sabin (blog|twitter)
• CODE TUTORIAL: SSIS Packages, Dynamic Parent/Child Variables - Justin Tubbs (blog)
• A deadlock was detected while trying to lock variables in SSIS - Darren Green (blog)

Even on Twitter, from one of the SSIS demigods:

The way those posts are phrased, it sounds like those methods are what you should use to get at SSIS variables when you're using a Script.  In my opinion, they're not.
The truth is that 99.9% of the time, getting dirty with those method calls isn't necessary. Using them requires you to understand what those methods do, what sequence they should be called in, and the fact you need to call Unlock after you're done.  (Oh, yeah - that wasn't in the list of VariableDispenser methods, was it?  Easy to miss...)
The reason those posts are out there is because the experts who've posted them know that most of the time, the simple way works perfectly, but they've run into a situation where it doesn't.  They're helpfully posting how to use the advanced technique - since the advanced technique isn't typically documented well.  But by doing so - and being very enthusiastic about the advanced technique, they (we) tend to lead others astray.  Additionally, they're confident in their abilities to use the advanced technique correctly every time.
The Simple Method

SSIS provides a really simple way to get at variables, and it doesn't take code.  See these two properties on the Script Task?

And these two on a Script component?

Quite simply - use those instead!  Here's the complete rundown.

Accessing Variables in the Script Task

Type the fully qualified variable name - or select it off the list exposed by the ellipsis (...) button - into either the ReadOnlyVariables property or ReadWriteVariables property.  Separate multiple variables using a comma (,).

Inside the code, those variables are now available in the Dts.Variables collection.  To read a variable called "StringVar", do the following:
// C# code to read an SSIS variable
string scriptVariable;
scriptVariable = (string)Dts.Variables["User::StringVar"].Value;

' VB code to read an SSIS variable
Dim scriptVariable As String
scriptVariable = Dts.Variables("User::StringVar").Value

If you have a variable you've placed in the ReadWriteVariables property, you can do the above to read the value out of it so you can use it in your script.  You can also push values into the SSIS variable, like this:
// C# code to write to an SSIS variable
Dts.Variables["User::StringVar"].Value = "New Value";

' VB code to write to an SSIS variable
Dts.Variables("User::StringVar").Value = "New Value"

Accessing Variables in the Script Component
Identically to the Script Task, type the fully qualified variable name - or select it off the list exposed by the ellipsis (...) button - into either the ReadOnlyVariables property or ReadWriteVariables property. Separate multiple variables using a comma (,).
The difference here is that accessing the variables is much more user-friendly... but where you're allowed to access them is not and I'll explain that next.  So first, to read SSIS variables in a script component, use code like this:
// C# code for reading an SSIS variable
string scriptVariable;
scriptVariable = Variables.StringVar;

' VB code for reading an SSIS variable
Dim scriptVariable As String
scriptVariable = Variables.StringVar

To write to a variable inside a Script component, do this:
// C# code for writing to an SSIS variable
Variables.StringVar = "New Value";

' VB code for writing to an SSIS variable
Variables.StringVar = "New Value"

Now for the "tricky" part in Script components... you can't use the above code just anywhere (nor should you).  The ReadOnly variables can be read from anywhere in your script - the PreExecute method is the most common place to read them, but you can also get at them in ProcessInputRow and PostExecute.  The ReadWrite variables can only be used in the PostExecute method - your script will report an error at runtime of "The collection of variables locked for read and write access is not available outside of PostExecute."  You can't even read them in the PreExecute method - all access to ReadWrite variables is limited to PostExecute.  If you need to read a variable's value in the script, do the work, then update the value, you'll need to use two SSIS variables to get that job done.
The Rationale for the Advanced Method
You've heard about the "simple" technique, and yet there are these fancy methods that other articles and posts may have said "work better."  It's absolutely true - but for a very small fraction of scenarios.  Here they are:
Tighter Control Over Locking (Concurrency)
When you place variables in the ReadOnlyVariables and ReadWriteVariables properties, SSIS writes code for you that you never see.  This code calls all those Lock* methods in the proper sequence and makes sure to Unlock them after you're done.  The code to lock the variables is run in the PreExecute phase of the task, and the unlocking is done in the PostExecute.  This means that for a long-running task (either a Script Task or a Data Flow Task containing a script) your variables are locked for a long time.  Is this bad?  Only if you have other tasks running at the same time that want to read or update those same variables.  They simply won't be able to do so, and SSIS will report errors.
Using the Lock* and Unlock methods allows you to reduce that window to only the portion of time that you need it.
This is usually a horrible rationale.  The fact that SSIS is reporting concurrent access problems with a resource should be drawing your attention to the fact that your package is improperly architected.  You have resources that two tasks are using.  One of those tasks will depend on settings made by the other task... but you want to run them in parallel?  Resolve the contention issue by serializing the tasks.  If you code the variable locking to a smaller area of code, all you'll be doing is reducing the risk of having a lock contention issue - you will not be eliminating it.  Your package will eventually fail in production - and you probably won't be able to replicate the failure to fix it.  If your intent is to have two script components in one data flow use the same variable - that's just bad architecture on your part.  There's absolutely no guarantee that one of those scripts will finish before the other starts - even if the data flow looks like it should happen that way.  Use the advanced technique in these situations at your own risk!
Reading From ReadWriteVariables In PreExecute
As you now know, placing a variable in a Script component's ReadWriteVariables property means you can only read and write to it in the PostExecute method.  Sometimes you may have a script process - such as a row numbering operation - that may need to read an "initial" value from a variable, run through the rows, then update the variable with the "final" value for later use elsewhere in your package.  In order to do this with the "simple" technique, you need to use two variables. 
Using the advanced technique, you only need one.  In the PreExecute method, call LockForRead and GetVariables, or call LockOneForRead.  In the PostExecute method, call LockForWrite and GetVariables, or LockOneForWrite.  Don't forget to Unlock the variable as well.
This rationale is understandable.  The alternative is maintaining two variables in SSIS in order to accomplish a simple task.  The downside is that you now have to understand what those methods do, and your use of the variables is not exposed to anyone who might support your package unless they read your script line by line.  Using the ReadWriteVariables property exposes your use of those variables very nicely to everyone else.  Using the advanced technique here makes for a cleaner set of variables in the package, so I can't say it's a horrible use case.
Best Practices for Reading and Writing Variables in Scripts
Given all those facilities and restrictions on using variables, and the freedom the advanced technique gives you - what should you do?  Here are my best practices for using variables.  They won't apply to every situation you run into, but you should try to adhere to them for the reasons I mention with them.

1. Use the ReadOnlyVariables and ReadWriteVariables properties instead of the Lock* methods.
   
   * Advertises to other package designers which variables you're using in your Scripts.
   * Guarantees that the variable locking is handled properly.
2. In Script components, read variables in PreExecute - not in ProcessInputRow.
   
   * Reading variables crosses the COM/.Net barrier and that's expensive.  If you need the value in the variable when you're processing rows, read it in the PreExecute method into a module-level variable, and use that.
3. In Script Tasks, read or write to variables once and only once.
   
   * The same reasoning as #2 - crossing that COM barrier is expensive.  Pull the value out and store it locally for use.  When writing, don't use the SSIS variable as an intermediate storage location - use a local variable and only assign the final value once.
4. In Script components, if you need to read a variable, process data, then write back to the same variable... don't.  Use two variables instead.
   
   * See the reasons for #1 - they apply here just as well.
   * Your SSIS variables will be labeled appropriately for "before" and "after" states.

Learn Something?
I hope you did - I know it took me quite a while to figure all that stuff out.  Some of the blame lies with guys like me - we tend to broadcast the "neat" and "difficult" solutions, and forget about the simple ones that took a long time to learn.