We're halfway through a garage addition/expansion project right now. We have really lucked out and have a great contractor. We were 2 weeks ahead of schedule and $3,000 under budget at the end of March. Then his wife delivered their second child...the nerve of her.

We're still under budget, but have fallen behind on the schedule.
Anyway, what has made our project so satisfying is the GC's no-nonsense billing method. He gives us all the originals of his subs' invoices and then just tacks on 18%. We interviewed 5 other GCs and he's the only one who bills this way. All the others estimated our project at $225,000+ and they wouldn't reveal one bit of their costs. Jack's estimate was $165K, and like I said, he's been right-on so far.
Another key to our success (so far, knock on wood, don't jinx us) is that we used an architect from the very beginning. Other than the obvious design and spec role, she does and excellent job of being the "bad guy" on construction stuff. (There really haven't been any construction issues, but she's can definitely spot stuff that we can't.) LVLs v. steel beams, 8.125" risers on the stairs v. 7.5", etc., etc.
Our project involves a complete demolition of the "old" 2-car garage. In its place, we're putting a new private entrance that leads up to DH's new home office. So, essentially, the new office is as large as a 3-car garage. The office has DH private space plus a "public" area for meeting clients, editing video, kitchenette, full bath, and lunch table area. Oh, and I get an 11 x 13 space on the ground level for a new laundry/mud room.
I've put a couple pictures in this album. The "before" shot was Jan 21. The other was March 30. We hope to have use and occupancy by Mother's Day.
Album:
http://share.shutterfly.com/action/welcome?sid=8CbN2rhuzasJo¬ag=1
As for any other tips, I have to agree with what everyone else has already said. I can't think of anything else, but will post if something comes to me.