Buy a beach ball, you will LOVE this exercise.
Sit in an upright chair so that your thighs are parallel to the ground. Put a beachball between your legs, about half between your knees, half between your thighs. Straighten your back, and squeeze your legs together as hard as you can for 5 seconds. Wait 10 seconds between each 5-second squeeze. I started with doing 20 of those squeezes on the first day, then added 1 second to each squeeze every day.
You will seriously get some awesome grip on the saddle after a couple of weeks!
Also, I'm not sure if you're a beginner, but if you're getting back in the saddle after a while, I think you'll immediately recognize that post-dismount pain between your thighs that makes you waddle like a penguin....the beachball will help loads, and at the same time will keep you one step ahead, especially if you're asked to post without stirrups or ride bareback. Keeping a strong core and seat is super important!
Also, squats. They help with just general thigh and calf muscle development. To work on keeping your arms and back in place, you can try tricep dips. This is what they look like http://www.womenshealthmag.com/files.../TricepDip.jpg
Also, tri dips are really fun because you can do a ton of them and not really get tired easily...maybe that's just me, but I always end up doing them at work when I'm on break!