I love doing Pilates. It's not really like yoga as everyone describes, I think it can just look like yoga to someone watching it for the first time.
I always notice a difference in just two weeks when I start going to classes again. It's not "whoa, let me go sign up to be a Victoria's Secret model!" but there is definitely a noticeable difference in my stomach tone.
You can sign up for a class (I suggest this because they person can correct your form, plus an actual class will keep you motivated to go). Or you can buy a DVD or look up Youtube videos