+1 on Pilates! Helped my posture, my back pain, AND it was great post-accident & post-PT.

I do mat as well as reformer (& other apparatus). I find they complement each other very well. I tend to get stuck in my mat progress unless I can also do some closed chain work on the reformer as well. Plus it's just great for variety.

Pilates is best learned in one-on-one sessions or small groups. It's pricey, but the best instructors are worth it.