Two words: Proxy Editing (aka the Bait and Switch Technique)
I actually don't like the explanation above because it makes it seem like you need to use avisynth scripts.
But in a nutshell, you just need to make a faster second copy of your footage to edit with. Then switch the files when you are done.
Sometimes you can just use an SD proxy in place of your HD original, and that speeds things up enough...
