2nd that, props to parker on this thread, i have been researching this heaps lately, mostly reading bmx racing stuff.
i read this massive article written by some pro bmx racer.
he said that if you're racing bmx/4x, the best possible training you can do is to ride your 4x bike at as many different tracks as you can, wearing most of the gear you'd wear when you race (as poser-ish as that may sound).
some people reccomend swimming, running, zumba! (lololol), but none of these can compare to riding your race bike on race tracks. Swimming etc are great of course, but doing that is like watching tv to get better at reading.
Also, make sure you practice riding the ENTIRE track. re-doing tricky jumps etc is great, but it takes a lot of energy to race an entire track, so it's important that you do as many complete laps as you can at full power.
another example that most people on here can relate to - it's like trying to get your best lap time in a certain car on Need for Speed, practicing in another car or on another track wont help anywhere near as much as trying on the same track.
Anyway in terms of strength i've read in a few places that the best thing you can do is squats, but rather than just lifting the dead weight, try to burst as you stand up, but lower the weight slowly.
this helps to build up that burst you need to get a good gate.
the only other thing i can reccommend is practicing your 1 handers, pushing another rider mid-air is the best way to slow them down.
anyway pretty keen to hear what else you guys reccommend.