I forget where I saw it, but someone was talking about installing onto SSD drives. They mentioned having all of their data on the secondary partition in addition to system restore files, page files and other system files that take up alot of space.
Personally, I don't really use the method above. I have a Macbook Pro w/ Windows 7 and OS X installed. I have a 50GB partition for OSX, and then a 250GB partition for Windows. In theory I could give Windows an 80GB partition, but I play alot of games and I don't need to store things on a secondary partition because I have multiple hard drives. All my data, videos, pictures, and such get stored on external hard drives. The only things that are on my main HDD are system files, programs and games. Anything I download gets stored on that HDD for a max of a day or two, then it gets either deleted or moved to one of my externals.
Oh yeah, I have to defragment once a week. Not only do I defragment, I use CCleaner to delete all temp files including system restore files except the latest. I also use my OS X install disk to repair disk & permissions once a month. If you do a clean install and then set all of your settings (including system restore) and then watch very carefully, you can usually tell where your disk space is being used. Some of the big users are your users folder, system restore, temporary files, and downloads.