The bushing:
Shrader valves require a larger diameter hole than do presta valves. Therefore, presta valves are kind of small compared to the hole in the rim designed for a shrader. You can buy little bushings the outside diameter of which is the diameter of a shrader and the inside diameter is that of a presta. Put one of these in a shrader hole and viola, you've got yourself a presta hole. One version of this is called "presta saver" and is like $2 to $3. Seems like a lot for what it is. Probably, the best thing to do is to buy shrader valve tubes, if you've got rims with those big holes in them.