I haven't been around on these forums for too long, but boy have I seen a lot of "My TempDB!!!" posts lately.

Your TempDB doesn't just grow because it feels like eating more space. It grows because it NEEDS to. If I was you I'd start by looking through SProc's, nightly jobs, any query that runs often that I didn't write myself. Look through them, you'd be surprised how in-efficiently some people can code. All it takes is one hungry query to bring your database to its knees.