1) Strap backpack on
2) Get on bike
3) Apply pressure to pedals
4) ???
5) Profit!

Seriously, though, I've used both a rack and a backpack. I started commuting with my old, Targus, laptop backpack that I had bought while I was in college. That pack was very uncomfortable due to its extremely high profile, its many flapping zippers, and the lack of a chest strap. My back would be covered in sweat even when it was cool outside. I switched to a rack and trunk bag. On my Giant Escape 2 hybrid bike, the rack/trunk bag combo was pretty good. The back end felt pretty well balanced. Handling was good too. I don't like the rack very much on my cyclocross bike that I got in November of last year. Because of the very light front end...it has a carbon fork...the back feels very heavy with a rack on it. I bought a Camelbak H.A.W.G. backpack on sale at Bike Nashbar and took the rack off my bike. The Camelbak is pretty comfortable. The straps are vented. The back has an airflow system which is nice, but is not perfectly sweat free. The pack has a low profile and is good in the wind. The only real problem I have with the pack is that if I fill the reservoir more than halfway full, my shoulders will hurt after about 20 miles on the bike. It's not a sharp pain or anything. It's just a bit of ache. I switched back to the rack for this week, just to compare. I'm taking the rack back off the bike tonight for tomorrow's commute.