How to be more flexible
How I did it: All that I did was yoga more routinely. I tried to do it every day, and what a world of difference I feel! I used to feel sore and stiff, my joints would crack, I'd have pain waking up every morning... even the beginner yoga stretches can really help, if you stick to it for a while. Even just regular stretching helps, every time before you exercise, or even a few minutes in the shower! (The warm water will loosen up your muscles if you feel stiff). I don't feel 100% flexible, but definitely more flexible. Feels great!
Lessons & tips: Start slowly. Do regular stretches, with or without exercise, and then increase stretching over time. Yoga is worth investing into, if you have the patience for it. Make sure you start with the beginner tape if possible, otherwise you might feel overwhelmed. I know the more advanced workouts go at a much faster pace, but you can find one that will work for you and provide great benefit.
Resources: A.M. Yoga Beginner DVD Kit - with Rodney Yee
