I'm still working out the plans/leveling the land for my own barn (six stalls, plus tack and a wash rack!), so I'll pass on the tips that I've gotten from asking around:
You know all those horse magazines that pile up at your local tack shop? Thumb through a few -- if you see anything at all related to a barn (building, rennovating, fire safety, etc.), even if it's just something silly like "spring cleaning your tack room", at least take down a couple notes. Also, if you know anyone with a barn, ask them what they like/don't like about theirs, maybe take a tour. You'll find all kinds of ideas that way.
Also, always make the building bigger than you think you'll need... it's so much easier to go back and add things than it is to put a whole 'nother wing on the barn! To save money, my current plan is to basically make a huge pole barn, and then just use that as a run in until I can save up a bit more money to add stalls, rooms, etc. My ultimate plan is to eventually have an indoor arena and everything, but my finances won't allow that to all go up at one time.
Also, make sure the horses can't get at the outside of the barn to tear it apart -- you'll spend more time fixing it than you will working with your horses!