Any backpack or bag can be used as a pannier for your bike. By adding something to keep the back pack out of the spokes and securing the straps to keep them out of the spokes you can save money and use something you already have. Also most panniers are not good back packs so it is nice if you are riding to take a hike to have a good backpack.
Step 1: Tube to Keep Pack Out of Spokes
I used an aluminum tube that is secured with hose clamps to my frame and rack of the bike. To protect the finish of your bike first wrap the frame with strips of used bike inner tube. Notice how the tube goes behind the frame and in front of the rack, this helped bow the tube away from the wheel.
A variety of different things can be used keep your pack out of the spokes. To test if it is going to work press it toward the wheel and see if it hits the tire easily. I have also used bamboo, however the bamboo eventually cracked and it was more work because I had to tie it together behind the tire adding more points to get loose.
Broken bike racks, rims and tent poles are all good materials to tie to your bike. Check your local metal salvage yard to see if what you need is there.
Step 2: Attaching the Back Pack
Now that whatever you found is secured to your bike it is time to tie on the back pack. My pack has a waste strap that I looped around the top of the rack and clipped. I tied the shoulder straps in front of the pack to keep them out of the spokes and I should have tied the end of that strap that is hanging all the way out of the photo. I used a clip to hook the top loop of the back pack to the top of the rack. Check to make sure it is not so close to the wheel or loose enough to hit the wheel while you ride. Add buggies or used bike inner tube to secure the pack and keep it out of the wheel and spokes.
Step 3: Keepin It Off Your Heals
Notice how the back pack is at an angle. This is to keep it from hitting your heals as you ride the bike. The pack should be on the rack and tube you added not in front of the frame. If your bag is too floppy and mostly empty cut a piece of corrugated cardboard, plastic, or anything ridged to the size of the inside of the bag and put it inside.