The general rule I follow is to use my unless there is a reason to use our (or use vars) to make package variables (A.K.A globals). There is only one reason to use globals: because you want to access them outside of their lexical scope. Reasons you'd want to do that include:

• You want to export them from your package.
• You want to allow external code to access them by giving a fully qualified name (this usually isn't a good idea).
• You want to use them higher up in the file than you declared them at (this usually isn't a good idea).
• You want to declare them within a block but use them outside of that block (this usually isn't a good idea -- though I can see it in rare cases involving BEGIN blocks).
• You want to access them via a "symbolic reference" (this usually isn't a good idea).

Note that using our won't let you do any of these except the first and last one.