A good starting point is getting familiar with the "heel-toe" technique. If you are unfamiliar with this here is a video-
its short but sweet.
You might also try an offset cam on your bass drum pedal- it may or may not increase the speed at which you can do them, but, I find that it really improves the crispness of the consecutive hits. For me (before I got the offset cam), I had trouble getting the bass drum hits after the first to have the same volume and crispness. Sorry if that went on, I tend to ramble on...