Contrary to popular belief, breastfeeding does NOT cause saggy breasts. I have heard countless women say this is why they chose not to breastfeed and it's simply not true. Genetics and pregnancy are what cause breasts to sag, so if you made it through your pregnancy with your boobs intact, you can breastfeed without fear.
A recent study in the Aesthetic Surgery Journal confirmed what lactation consultants have been saying for years. The most significant risk factors for sagging breasts appear to be age, number of pregnancies, weight and a history of smoking. Another reason to quit smoking now if you haven't already.