Try military style pushups, where your hands are further under your shoulders, back towards your ribs, don't let your elbows flare out on the way down and up, sort of like a sun salutation.
Second, try pyramid pushups, where your thumbs and pointer fingers touch in front of you.
Third, try dive bomber pushups, start in down dog, and "swoop under the fence," ending in up dog, then reverse. Dive bombers sure get your heart rate up.
These three pushups are harder than regular pushups and fry your chest more.
The P90X dvd's give a pretty good idea of a lot of the different pushup styles.
You can also go to
www.ronjones.org, he has free handouts on most of the pushup variations out there.
If you want further tips on different types of pushups, calisthenics style, in circuit format, look up
www.sealtraining.com. Scott Helvenston, a former Navy Seal, has some killer calisthenics style workouts on dvd with lots of different variations, no weights at all. He died in Iraq, one of the four civilian contractors who was shot and killed (defending a convoy, they were overrun) and whose body was dragged through the streets and hung from a bridge. His videos live on, a tough testament to his weights-free fitness.