Push ups on your knees is a good place to start without straining your back.
Lately I have been working on a series of articles for various health and fitness magazines about training after having a baby or trying to get back into peak performance shape after having a serious injury.
The first article I wrote about the importance of strengthening the core and the chest for framing. Ladies, it is very important that your core is strong before you progress to doing push ups on your feet. If you can perform 20 push ups on your knees with a straight back (no banana back) then you are ready to progress to push ups on your feet. But if your back is sagging, stick to push ups on your knees, aim to get your chest almost to the floor to get your core strong, otherwise you might straight your lower back. Baby steps.