Just borrow some kid's school backpack, adjust it to fit you as best you can, chuck a bunch of clothes and what-not into it and go for a ride. You'll soon discover that backpacks suck - especially when standing to climb or sprint away from a light.

Then mount a rack on your bike and get a trunk. The nicer ones have pockets on the sides and are even expandable, so they really do hold a lot of stuff. Some trunk/rack combo's are even engineered with quick release mounting. Very nice, but not absolutely manditory.

If you really need to haul a lot of stuff - say, a change of clothes, lunch, your laptop, maybe some cold weather and/or rain gear - get panniers. Panniers are also nice in that they maintain a lower center of gravity than trunks when chock full of yer stuff. You can feel the difference if you stand when climbing.

I have both, but try and pack light enough so the trunk will suffice. If you can't afford both, get panniers. There's no law that says you can't mount just one when you're travelling light. And you won't be thrown off balance when running just one - unless you pack your bowling ball and shoes in it.

All the best. DanO