dont reset it while you are moving. the mpg reading will show some weird numbers depending if your accelerating or coasting. if you reset it and then coast down a hill, you can get it to hit 99 mpg.
edit: there is a thread around here with pics of a 99 mpg reading
best bet is to reset it just before you leave the pump or put it into drive. if you reset after you leave or when you are at a complete stop later on, you will still get weird readings.
i reset my mpg after every fill up and just before i put the vehicle in drive. i consistently get around 14-17 mpg readings after about 1-2 miles and for the rest of the tank.
YMMV