Need opinion on diet?

First of all, my diet is not intended to lose weight, its purpose is to keep me healthy and help me tone muscle. I am about 105, and flactuate between that and 95 lbs unintentionally. I have had 3 beautiful and healthy daughters, gained up to 65 lbs, and lost it every time. I just dont seem to be able to keep weight on. So, here i am asking what the best ways to build muscle are. I have been cutting my soda consumption, and i am down to 1 to 2 cans of coke per day. I have replaced my liquids with an average of 1 gal of water per day, plus i have added 1 serving of ensure per day. If this helps, i do an intermediate routine of exercise before power walking a minimum of 30 minutes. If you can tell me what has worked for you, or you are an expert on the subject, i would love to read what you have to say...thanks!