Read this book, go to craigslist and volunteer your time for student projects. Experiment like crazy. Or you can take the college route. Try watching movies with the image off and only the sound on. Try listeneing to every sound on the film. Most movies have a 100 layers of sound. (Collateral did)
Also check out the film The Great Raid. It has a special feature where you can activate different tracks of sound during the shootout at the end. It's kind of neat. On one your watching the shootout and only hearing secadas. On another, you only hear Japs going "arrrrhhh" when they get hit. Another is all gunfire, ect.
They say if nobody notices the Foley then you've done a good job. On films where there is no Foley, you sure can tell it ain't there. When it's there, you don't notice it.
I filmed a scene a few months ago where we had two guys in a restaurant talking. There were no extras because of a snow storm so we filmed it with just two actors in the restaurant. I added in people talking, fork clinking, plates clanking, steak frying in a pan, and a piano playing (but I crappened up an mp3 file to sound more like it was in the background rather than the soundtrack, this is called "worldizing" the music.). It sounds pretty good as if there's a crowd of people in this restaurant.
If you don't feel like doing the college route, experiment, experiment, experiment. And horror movies are your best way to practice. Take a good horror film, get rid of the sound, then put in every sound yourself to practice. (just make a trip to the farmer's market for all your horror film Foley needs.)