I finished (well, mostly - still working on the trim) a bonus room above my garage. If your bonus room is above a garage and your garage isn't heated, you might check whether the floor's insulated - mine wasn't. R19 in floors, in case you're curious. I second that it's an inefficient space, I wish I'd put in more insulation. Also might want to check your heating system to make sure it's sized for the extra space, my room's close to 400 sqft and makes a noticeable difference upstairs.
Other things I learned:
You might want to check on the circuit load - I had to add one since it shared the upstairs circuit with two bedrooms and a bath, and I wanted a bunch of lighting and cool toys up there.
+1 for hiring out the mudding, it's the world's worst job. Plus lots of guys need the work right now. Did it myself, and really wish I hadn't - hate to think how many hours I wasted fixing mistakes after the first coat of paint highlighted them.
To save money, don't just order in the sheetrock, order the speedbase and other trim too, and don't forget the screws for the sheetrock. Most building supply places beat the BORG every time.
Don't forget those little metal stud plates that cover wires in the studs before you put up the sheetrock. Inspectors don't care for that, it turns out. :BangHead: