Tina Haupert at Health.com writes here about adding weight/circuit training to your cardio routine. It can slim you down because your body keeps "working" even after you've put up the weights. (Don't ask me exactly how this works.)
Weight training builds muscle, rather than fat, as well. I myself have gotten out of my weight training. It just happens. I love my Spin class so much, and riding outside, that I neglect the weights.
Even if you don't have a gym membership, you can find at-home circuit training routines online.