Take a look here:
http://www.stumptuous.com/cms/displayarticle.php?aid=75
Mistress Krista tells all about how to develop a pushup.
One difficulty many people have with pushups is the core strength to keep ones body straight. You can try doing planks, on elbows, then straight arm, then eventually in the down pushup position. Shoot for 1min in the position.
If your only difficulty is with pushups, I would substitute pushups for any horizontal push movement until you can do at least fifteen. Twenty is better, however.