Commuting - How do I load my non-commuter bike?

I commute to school with a backpack on my 2009 Kona Paddywagon. My bike is substantially less fun to ride with weight pushing me down while I ride in the drops and straps constricting my breathing. I'm sick of the large patches of sweat wherever the bag touches me, and I dread the bulk of my bag on top of my heavy winter coats. I want to put it on a rack.

I've heard the old Paddywagons had rack braze-ons, but they stopped using them. Should I worry about overloading my rear triangle or damaging the tubing with p-clamps? I'm also concerned about loading the weight of all my school books on one side with a pannier or backpack-pannier. I originally thought to just get a waterproof backpack or messenger and bungee it to the top (or use my current bag wrapped in plastic). Is this the best plan? Are there large trunk bags (or something that sits on the top of the rack, rather than the side) that are meant to be detached and carried the same way as the Ortlieb and Arkel combo pannier-backpack/messengers?

I'm not interested in buying a new bike yet.

Thanks for reading. Thanks to anyone who replies, especially if you're good looking or helpful.

If you have too much weight for one pannier you can certainly use two. Trunk bags generally have a smaller capacity than a pannier. Or you can get a commuting bike. You need fenders as well as a rack. N+1, baby!

I have a weird, old rack that screws into braze-ons near the seat post, but then uses clamps to attach to the rear triangle, above the wheel. It's rated for 55 lbs; if you could find a similar rack that also clamps to the seat post, like clip on fenders, it should have a decent weight capacity.

How much does everything you want to carry weigh? Books are heavy, but how many do you bring with you at a time? I've been carrying a single book and a few other things on my rack, in a kayaking dry bag for the rain.

Just looked up the paddy wagon: Shame on Kona for not including proper braze-ons for fenders and racks on what is obviously supposed to be an "urban" bike.

I wouldn't worry about P clamps damaging your seatstays. for the mount points near the hub. for the mountings near the seat cluster, use either a seat post collar with rack mounts, or just the mount that goes around the seatpost, but doesn't secure the seatpost to the frame itself. be sure to get a rack specifically for short chainstay bikes that allow you to mount your load farther rearward to avoid heel strikes.

I'll let others chime in on the wisdom of using p-clamps, etc, but I'll throw in my vote for just bungeeing your backpack onto the top of the rack. I guess some of that depends on how much you put in there & how much it weighs, but I use my pack this way every day. The advantages (as I see them) are 1. the weight is distributed right over the wheel so it's pretty even & 2 you can use your backpack when you are not on the bike. Lugging panniers around off the bike is a pain for me. If you use the backpack off the bike much, the versatility of a backpack is something to consider.

Thanks for the replies. I'll see if my mechanic can hook me up with a sweet rack.

I use my pannier loaded with school books and a laptop hanging on the non-drive side of my bike. I estimate that I have often carried 20 pounds in the bag and the only time I notice the bike being off-balance is during mounting/dismounting. I have used p-clamps successfully over the years without causing any damage to my seatstays. One of my bikes has no braze-ons for the upper rack mounts so I use one of these seat post collars (as was suggested by a previous poster):

http://www.adventurecycling.org/store/images/products/AxiomTrekk%20xl.jpg (http://www.axiomgear.com/products/gear/racks/rack-accessories/trekk-seat-collar/)

Here is my rack mounted on said bike:

http://imgur.com/FepWYl.jpg