LinqToSql declare and instantiate DataContext best practice?

As Rex M said, the datacontext is intended to be instantiated, used, and disposed for each logical transaction. Patterns like this are sometimes called a "unit of work".

The most common way (that I know of) to do this is to instantiate your datacontext in a using block. I haven't used VB in a while, but it should look something like this:

Using dc As New MyDataContext()
   user = (From u in dc.Users Where u.ID = UserID).Single()
End Using

This not only reinforces the look of a transaction/unit of work (through the physical shape of the code), but it ensures calling Dispose() on your datacontext when the block ends.

See this MSDN page:

In general, a DataContext instance is designed to last for one "unit of work" however your application defines that term. A DataContext is lightweight and is not expensive to create. A typical LINQ to SQL application creates DataContext instances at method scope or as a member of short-lived classes that represent a logical set of related database operations.

First, make sure you are disposing your DataContext when you're done! He can be a heavy little bastard (edit not heavy to instantiate, but heavy to keep around if you keep using it without disposing); you don't want old DataContexts hanging around in memory.

Second, the DataContext is intended to represent a single logical transaction. E.g. you should create a new one every time you want to start a new transaction, and get rid of it when that transaction is complete. So for your purposes, that is probably the scope of the GetUser method. If you have a series of DB calls that need to be made as a group, they should all use the same DC before getting rid of it.