reducing size of partition

I've been using parallels for over a year on my macbook pro so I could run XP and use it at work (wintel world)

My partition size of 31GB was recommended, but I am only using about 10GB of that space and would like to reclaim it for use on the mac size. (since the mac size is filling up). I don't store work files in the partition itself - so I don't see my need for space significantly increasing any time soon.

any way to reduce the size of that partition easily?

I've seen some posts to suggest partition changes can require a full backup and reformat of the drive. So let's say I back it up, reformat, and reinstall the mac side. Then I create a new smaller partition. And how do I then reinstall my virtual machine onto that now smaller partition (when time machine backs it up, it backs up the 32GB file everytime, not individual file changes, so it isn't going to fit onto a smaller partition)?