From this side of the river, Park Express on 170 is where I & all my coworkers go. Do a search on Google for a $2 off coupon. Just under $20 for 2 days when I parked there in December (I hadn't flown in over 10 years prior to that).
If you are coming from the east or via 170, Park Express it is. It's on the corner of 170 and Natural Bridge, and it is the best-run airport garage I've ever seen anywhere. They have free drinks, too. Sign up for their card and they will give you a two-week freebie for every 10 separate times you have parked there. (Sometimes we have to park 2 cars so we get double credit.)