I'm a big fan of keeping the pace up in my RPGs. I don't always do it, but I prefer it when I can make it happen. I've done everything from 3-2-1-action counts in combat to real-time decision making to forbidding rules lookups and using stripped-down rules and resolution to keep things going.
Sean over at Power Score has a nice post on pacing games:
Dungeons & Dragons - Pacing Your Game
His games seem more scripted and plot-arced than mine, yet the advice is the same. It's a good bit on how not to get bogged down or let the game get bogged down.
And yes, he's spot on - you have to do a lot of prep work to be able to improvise.