Most women add push-ups to their regular routine as an afterthought and then wonder why their upper body isn't looking better. As with any other muscle group, you need a sufficient level of exercise variety and weight resistance training in your program.
A combination of bench press, incline bench press, cable or dumbbell flyes and push-ups is the variation and difficulty you should be aiming for.
Once you reach a certain level of fatigue, push-ups aren't going to do much for maximizing strength gains. Unless you put a plate on your back or find some other way to add resistance to push-ups, they're not going to help you get stronger.
Instead, add them to the end of your workout as a finisher.
