I have ran into this a few times where you get put on a project and you want to do unit testing of some type, but the project isn't setup right to allow for this and you might not have buy-in from everyone. The main issues tend to be code smells like issues with dependencies (especially the over use of statics) and seperation of concerns. New code that you write can be setup easily, but anything that operates with other legacy code or even those not drinking the cool aide can become an issue.
Some things that I have found are that isolation frameworks like TypeMock can make this easier. What are some other tools/tricks/best practices that you have found that ease this sort of pain?