You could try getting brake pads that don't suck/are not dried out... Kool Stop Black pads are impressive, but Salmon are really great. Your LBS may not hold 'em in stock, though they could order some for ya. They look like these:
Or, if you have not so good V's that don't even have holder/refills, then something like these will work. Though last time I checked for pads I saw in the catalog that the ones that looked identical to these were called MTB, not BMX:
If your cables are old or not oiled, too long/short, too many bends/sharp angles, if the V arms need grease, if you never get your rims even a little wet while riding, they can accumulate a sorta greasy film. Bad pads won't ever work very well, old pads get dry and hard, should be replaced even if they're not worn out. These things will kill off braking power. A fresh set of cables can make an impressive difference, and the salmon pads above are divine... Brake pads are cheap and quick/easy to replace.
If the brakes are mis-adjusted, if the cable ends were not cut flush during install, if ribbed ferrules were not crimped (or pressed in real tight) on the cable ends, cables too long or too short, routed so they bend sharply, old/dry cables, plastic parts anywhere in the brakes, bad brake pads, all of that contributes to spongy brakes.
How-to's and stuff:
http://www.parktool.com/repair/byregion ... ield2.y=10