Here're my best practices to help get everyone started. If you're moving from XP to Win7 and thus doing a clean install, or if you're getting a whole new machine:
1) De-authrize your old machine via iTunes > Store > De-authorize computer
2) Get a USB drive of sufficient size to copy all your iTunes media (much easier done if you're letting iTunes organize everything under one iTunes folder).
3) Copy everything over, the entire folder tree, everything (don't forget your documents and other data, I'm only talking iPhone/iTunes here!)
4) Install Windows 7 (or setup your new machine)
5) Install iTunes
6) Authorize your new machine under iTunes > Store > Authorize Computer
7) Copy all your iTunes content from USB drive to Windows 7, same directory structure
8) Restart iTunes (if it doesn't detect your library XML file, you might need to trouble shoot a tad)
9) Sync your iPhone. (I'm almost certain you will need to re-select what you want to sync over -- any Windows gurus know better?)
Any more/additional/better advice welcome!