If you have a partner to help you. Try manual resistance flys. Perform as many as possible to complete failure and then do your pushups. May want to take a day of rest after performing these.
Manual Resistance Flys:
Start lying on your back on the floor or a bench and perform a fly movement while your partner places resistance on the way up and the way down. They usually place hands on the forearms for effective resistance. Keep movements slow and controlled. Don't pause at the top or bottom of each repetition. I prefer to reach failure in about ten reps.