Manually manage music on iPhone between multiple pc's ?

I've been using manual manage for all my ipods for years, and I've tried to select the manual manage song option in iTunes with my 3G iPhone. It works fine for one computer, but I travel between work and my girlfriends place alot and can't add songs on those computers.

Everytime I load up itunes on one of those computers, the manual manage songs option has been unchecked and if I click it and hit apply it says its going to erase everything because its synced to another computer.

Any ideas what I might be doing wrong?

this worked for me:

http://www.andrewgrant.org/2008/03/30/how-to-sync-an-iphone-with-two-or-more-computers.html