To straighten your back, don't push your shoulders back, or thrust your chest out - rather, pull your shoulders back. There's a couple of muscles between your shoulder blades and your spine, and consciously tighten them for a few mins each day (I do mine when I do my pelvic floor muscles). And just like your pelvic floor muscles, if you exercise them, you'll be doing it a lot more.

When you use these muscles, you can actually feel your rib cage lift up and more air getting into your lungs, and your diaphram (sp?) moving too.

Plus yoga / pilates is wonderful