I'm no pro, but I've run my share of AX, so here's my $.02:
Bike:
-Slide forks up in clamps
-Move rear wheel as far forward as possible
-Add 2-3 teeth to rr sprocket
-Siffen suspension
-If the track's really tacky (i.e. you're wheelying really easy), add a couple pounds of air to your rr tire
Practice/Training:
-Train for short sprints. Short intense jump roping, sprints, etc.
-When riding, go hard for short bursts to work on speed and aggression
-As boomer said, practice STARTS
-If you don't have access to an AX/SX style track, shorten your outdoor practice track. Pick a few straights with jumps, and connect them with tight 180* turns.
In all reality, AX is about aggression and timing. Bike setup and fitness (while still important) play less of a role.
Good luck!