Well, there's an article that addresses this very question here. The short version is that OSX's Disk Utility is your friend. Use it to alter the size of the OSX partition, and then re-adjust the Windows partition.
Just remember to do a clone of your drive before you do so - you might just stuff it up, so it's always great to have a bootable backup. You can use a few different programs to do this; I've always used Carbon Copy Cloner. It used to be free - it now isn't - but comes with a 30 Day trial period. (As an aside, I've used this program heaps of times, and it's totally worth the price of entry!) SuperDuper! is another - never used it, but you can give it a shot.
Make sure you don't delete or alter your MBR! That's when booting issues happen - you'll remove your Mac's ability to find the bootcamp partition (hence the importance of the cloned backup - gives you somewhere to go back to!)