Check out this blog post from the Powershell Team. It shows how you can make variables in Powershell behave as global static properties. Be sure to grab the New-ScriptVariable script.
Note that it uses the Add-Type cmdlet available in Powershell 2.0. If you don't have version 2.0 then Add-Type can be replaced with my New-CAssembly script. It's a really easy conversion. In fact I already converted it and is available from my SkyDrive.