Don't forget the return.
If you throw your punch fast don't leave it hanging out there, the sooner it's pulled back the sooner you can throw it again.

I think this is the book I have that had some good tips in it.
Speed Training: How to Develop Your Maximum Speed for Martial Arts

Don't forget it's not just being able to throw fast punches if you don't train being able to see the openings and it's no good being fast if your accuracy is off.