How do you know you have a memory problem? I've just tried rendering a bit of video with loads of transitions and slo-mo - the memory usage barely increased.
I don't know how to reduce the amount of memory VF uses, so I'd aim to reduce the usage by other programs. Start by getting hold of 3 freeware programs - Spybot, Enditall & MaxMem. Spybot checks your system for spyware and will give you the option of getting rid of it.
Enditall tells you what programs are running and gives you the option of killing off anything you don't need. AV software and firewalls are good targets, but make sure you're off-line first! A friend of mine has defined a rendering profile on his system so that the drivers for the nic and any other devices he's not using aren't even loaded.
Finaly, MaxMem recovers unreleased memory, so there's a chance you're not wasting any.