You can use something like this:
http://www.google.com/products?clie...esult_group&ct=title&resnum=3&ved=0CB0QrQQwAg
I think doing it w/o some kind of adapter is a bad idea since the tube might squeeze through the space by the valve and do bad things. Having said that, I've done it once in a pinch and road a couple miles.
I've had multiple flats using presta valves in schraeder rims, tore tube at base of valve stem
Can anyone give me a list as to why Presta is that much better than Schrader valves for mtn bikes? On every bike save for my road bike I have Shrader and love it for the convenience. There is a Shrader/auto nozzle in my garage and I can get the tire pumped up quick. If I'm someplace without a pump I can just go to the local gas station for air and not worry about carrying an adapter.
Presta are more dainty, and as such, go better with shaved legs and matchy matchy ballerina outfits.
I prefer schrader as well. I've had several of the nubs on presta valves break off in my many years, and don't recall ever having a valve malfunction with schrader. They're simple and durable. All you need to do is remove the valve cap to pump them up. Presta, you have to remove the valve cap, loosen the nub, then carefully attach the pump to avoid bending the nub.
Presta are more dainty, and as such, go better with shaved legs and matchy matchy ballerina outfits.