Environment specific variables

I would recommend using Custom Settings (read more at http://na1.salesforce.com/help/doc/en/cs_about.htm). They basically work as global variables for your orgs and can be set independently in production and in sandboxes.

Custom settings come in two flavors: a list and hierarchical. If you need to vary the setting by role or profile you would want to use the hierarchy option otherwise the list option is very straight-forward.