I've been playing double bass for about 2 years now. I'm not even going to begin to say that my double bass drumming is excelent but I have definatly noticed an increase in speed and comfortability while playing double bass. Many of the fills I use these days have double bass in them, much like the mike portnoy fills.
Anyway an exsersise that my drum teacher wrote out for me wayback then is this:
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It definatly helped me increase my speed in drumming. It was very useful and also helped me become more comfortable on it.
I suggest you start of very slow when doing this exercise. It is very difficult to play anything past six tuplets at even 120 bpm.
I usually can't play everything in this exersise but it definatly helps. Practice it over and over.