I have a couple of messenger bags that I don't use very often (coincidentally, i also live in San Francisco). After some doctor-says so, I decided to lighten the load on my back and shoulders so I converted the bag to a pannier. The whole process took about 1 hour and cost me less than $30. The project uses only basic tools and all materials can be found in your local hardware store. The grommets are actually carried at my hardware store but if you can't find them, try a craft store. I started by making only one pannier. I actually find it very easy to ride with just one, even when it is weighted down. You could always make a pair if you like feeling balanced. It is also designed to be easy to snap on and off so you can carry the bag with you. This is roughly based on other folks ideas but I tweaked it with the addition of grommets, rope clips, and the "chain quick snaps". I've been using the bag for about a month and it is working perfectly. Also, my landlord says I look cool.
Step 1: Materials
One messenger bag (re-purpose an old bag!).
3/4" Stainless steel machine screws (#10) - 2 $1.50 ea
#10 Stainless steel hex nut and washer- 2 - $0.50 ea
3/16" rope clips - 2 $0.88 ea
Grommet size "0" - 6 $3 dozen
Grommet size "2" - 2 included with grommet tool
Blue Hawk 3/4" Chain Quick Snap - 2 $7.00 ea
One bungee cord (24" length) - $1
One key ring (about 1") - $0.20
1/4" Plywood - 13.5" x 17" (may be smaller for some bags) - $5
Saw (handsaw, jigsaw or bandsaw)
Hand drill and bits
Total cost: $29
Obviously this would be cheaper if you already have some unused bolts or plywood in your garage. You could also save some $ by using a carabiner.