I'm tired of numbers and calculating gear inches every time I need/want to change gearing for riding trail. I finally made this little cheat-sheet of some standard gearing I run on both 26" or 29" mountain bikes. Translates like this:
(Front Chain Ring) / (Rear Cog) = (Gear Inches)
For typical local riding I try to stay as close to but not over 50 gear inches. For riding trial with lots of climbing or technical terrain, I might back down from 50 a bit. For double track and cruiser stuff you can go over 50. Play around and see what works best for you.
Don't trust my math? There's always Sheldon Browns Gear Calculator HERE if you want more info...