wear natural fibres, like cotton, linen, pure wool, etc. synthetics like polyester, nylon and such don't breathe and will make you sweat more, and won't let the sweat evaporate off as easily. check the tag while you're shopping -- if it doesn't say "100% cotton", don't buy it.

shave your armpits (or wax or whatever you like). it'll be difficult the first few times, and will take a while to get used to (itchy and stuff) but after a few weeks you won't even notice anymore. there's a couple of advantages to this: the first is that it won't get so warm in there, so you won't sweat as much; the second is that the sweat and smell will evaporate right off your skin instead of being absorbed by the hair, so if you do get sweaty for some reason (running for a bus or whatever), once you've dried up you won't really smell anymore either.

drink a lot of water. i have no references to back this up, but my own experience suggests my sweat is less smelly if i'm well-hydrated. if i've been drinking coffee/coke all day and not backing it up with a lot of water, by the time i get home i absolutely reek. if i'm drinking water though (and i usually drink a couple of bottles of water a day), i'm fine.

stick with anti-perspirants rather than simple deodorants -- deodorants won't stop you sweating or smelling, they'll just make you smell more strongly of something different to hide the odour, which is really annoying for the people around you. if you're feeling adventurous, double up with an unscented anti-perspirant and some subtle and pleasant scent (cologne or whatever you like).

if you're already doing all these and it's not helping, see your doctor. they may be able to prescribe some anti-bacterial medication or something.