This is kind of a fun variation on push-ups that I used to do. Get a partner, and get into push-up position, facing each other. Pick a number, like 5. One person starts by doing one push-up, then the other does one, while the previous person waits in push-up postition. Then the person who started does 2, then waits while the other does 2, and so on up to 5; then, you work you way back down. It is a good drill, and it makes you hang in there and push each other.