From a technical standpoint, what do you think it is that makes each book so fast paced?
So far I've noticed:
Constant danger: There's always the threat of something going wrong, which keeps you on your feet. Time seems to go fast when you're having fun or anticipating something, so scenes never seem as long as they are.
Spectacular fight sequences: The danger regularly tips into a scuffle or a major fight sequence. These look cool and have consequences, so they're both instant gratification and full of tension. There are a large number of them in each book (relative to most other books) so by the time you get to the end you feel like you've had a satisfying amount of adrenaline injected into your system. The fact that there are so many means that you know there's something explosive around the corner -- you just don't know when it will jump out.
Switching between important subplots: This provides variation and adds to that feeling of anticipation because you know all these issues are up in the air and they WILL come down, with consequences. The fact that they're important means that the way they pan out matters, so you care. They also tend to feed back into the plot -- now or later -- so the consequences are generally long-term.
Rate of revelation: Each scene introduces something significant about the main plot or one of the subplots, or both, so it feels like there's constant progression. You're not waiting for things to get moving again.
I've been using these in my writing for a while, because for years I noticed them consistently in all of the action-packed stories I've enjoyed. Thinking back over the Dresden Files made it absolutely clear to me. Pick up on anything I've missed?
PS: I don't encourage checking your story against these while you're writing because it'll most likely stifle your creativity, but it's helpful as a checklist... well, it is for me, at least.