What height, weight and dress size are you? Do you do a lot of exercise? After I had my first DC I lost a stone without any effort through breastfeeding and exercising. Have managed to keep some of it off through exercise. Suppose what I'm saying is that running around after kids obviously means that you get more exercise and therefore it can be harder to keep weight on.
Nomadic I have a slightly different issue in that I have been overweight and now have actively lost weight. I love my curves so am a little sad that my bottom is deflating. So my action plan is to do some weights exercises using bodyweight and dumbbells - squats and lunges are very good for firming up and building the buttock muscles [smiles]. Also if you have access to hills or a treadmill then I'm finding walking up a steep incline really helps boost my bum!