I too had a rude awakening when my perfect breasts did not regain their orginal shape after a long stint of breastfeeding my child. Of course, I do feel it was worth the effort and was glad to do it. That being said, I was still eager to find a way which was non-invasive to tone and strengthen the breast muscles.
What I found to be very helpful is an exercise called the Deer Exercise. It is incredibly easy to do and although my breasts never looked like they did originally, I am very pleased with the effect of my diligence in doing these exercises daily. I only do 50 repetitions and do it in the shower or bath using a natural soap or oil. It takes a few minutes but should keep my breasts looking good for a long time to come.
Here is a good link which describes how to do the exercise. I wish you luck in your effort!