Bike panniers--commuting essential, but can be "beaucoup expensive."
For all of you on a college/unemployed budget (or just into recycling/reusing), here are easy plans for recycled panniers.
2 oversize leather bags/purses (thrift store/great aunt's closet)
2 dowels (wood, aluminum, anything stiff enough to keep the bags from hitting your wheels)
speaker wire (scrap wire etc.: check trash, recycling, re-stores etc.)
1-2 belts (cloth)
leather hole punch
Step 1: Punch the Holes
-Holes are ~1/2" (at least) from top edge so the holes don't rip out the edge under weight
-Equally space the holes. I recommend ~1.5" between holes
-There are equal numbers of holes on both bags
Punch out the smallest size holes you can get away with, where you marked. Your scrap wire should be able to just fit through the holes.
Step 2: Thread the Bag
Leave extra room between the two bags since it's easier to tighten the wire than put in slack later. The panniers will rest over your rat trap (the wire lays over the rat trap, the bags hang down the side), so "extra" room is anything wider than your rat trap.
When you've made it through all the holes, position the bags where you want them to sit (over the rat trap). Make sure they're far enough back that you don't kick them while pedaling.
Begin tightening the wires from the middle out, so the bags are snug against the edge of your rat trap. Once you're satisfied, tie off the ends TIGHTLY (I used a double figure eight with two safety knots).
Step 3: Buckles
Sew over the free end of both belts, leaving a pocket big enough for another wire piece. (Make sure the stitching is strong--this will bear a lot of weight also.) Cut off the buckle end of the second (longer) belt, and finish this edge by sewing it over.
Punch two holes on the outer side of each of your bags, 3-5" from the top, mid length. The holes should be as far apart as your belt is wide.
Thread a wire piece through the bag holes and belt "pocket," again tying the wire securely.
Step 4: Dowel Wheel Guards
Cover dowels with one layer of duct tape to keep from scratching your frame.
Attach dowels to the bottom, inside edge of each of your bags using more scrap wire (again, punching holes in the bags just as wide as the dowels and tying securely.
Step 5: Load 'em Up!
-Panniers can be tied down to the frame at the dowels. If you plan on taking them off often, velcro straps might be worth it.
-Use old plastic rice bags or bird seed bags to keep things dry that need to stay dry.
-Fashion first: do it up. Homemade panniers are great places for bumper stickers, art, mantras and so on that you might be too bashful to put on your $150 bags.