I have and highly recommend the 5.11 72 hour pack. Lots of storage for longer trips and super comfortable.

 

I use a Camelbak and it has a waist strap that keeps it close to your body.

I think the waist strap is key.

 

I didn't know backpacks were in style? I just use one for commuting to work on the bike since everything I take is coming inside with me anyway. Either way, if I ever get to the point where a packed lunch and a sweatshirt will screw up my back, I'll either hit the gym or take up quilting.

 

If the OP isn't using a sissy bar, The T Bags Raven tail bag would work fine. I've used one for 3 years, and it's a well thought out, well made piece of gear. The optional top bag can be used for a small amount of additional storage, and you can also cinch your jacket between the two pieces.
http://www.tbags.com/raven-soft-bag.html

 

Real men aren't afraid to use a backpack on a Harley.
Heck, I've even ridden with my wife's Barbie backpack just for fun.

 

This one is somewhat less offensive.

 

Yup. Worn a hello kity bag once or twice myself.

 

If you are going to wear a backpack exclusively for riding a motorcycle, the best brand designed for that purpose is going to be Kriega. Most packs, even the pricier ones with better harness systems, are still not designed with the motorcycle riding position in mind. These are. They are pricey, but the quality and comfort is top shelf.

http://www.kriega.us/r25-motorcycle-backpack/

 

I use a jansport that cinches down& has a waist strap and fairly wide shoulder straps. IMO those are the important features you want if riding any distance with one. The Kriega that hammerdown77 mentioned looks to to have nailed it.

 

You all are saying I'm supposed to conciencious of my style when I ride my Harley??...haha.
I've been known to rock a backpack. No idea what kind though.....it's blue. Works for me when I need to carry some extra stuff.