Here's how I got into great, firm shape: Ballet. Maybe it's not your thing, but it really works. I'm not saying you need to aim for New York City Ballet, but it's way, WAY better than aerobics and weight training. Why? Because it works your core and all the little muscles you didn't know you had, trimming all angles.
Seriously, I used to go to aerobics 5 days a week and got nowhere but bored and unmotivated. Then I thought I would give ballet a shot. Keep in mind I was 25 when I got back into it and hadn't danced since I was 12. I found a class for adults, but I was very awkward at first. But I got better as I learned how to move my body differently. 4 months of ballet (3classes a week) did more for me than all the boring aerobics and weight training. Everything got firmer and my thighs really slimmed down as I built muscle. Once I cut out granola bars, I started loosing weight.
Idea of dance class a nightmare? Swim laps instead. It also strengthens your core, is aerobic, and easy on the joints.