Improving flexibility?

This question is concerning flexibility, when punching. I have good hand speed, however it takes me a while to get loose enough to throw with my natural speed.

So I wanted to know if there are any exercises to loosen up the shoulders a bit so you won't be so stiff. I know there are exercises to improve hand speed, like shadow boxing with light weights, shadowboxing, etc. However, do you know of any exercises to improve flexibility in the arms, particularly the shoulders, short of stretching?

Honestly,you just might be a slow starter. If your not allready sweating when you go in,its going to feel like your swimming through mud for at least a round