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).