+1 on ParknFly! I have given up on GTAA parking, too expensive, and you walk a mile! With ParkNFly, my walking from car to terminal and back to my car has been reduced considerably, and it is cheaper!

Another vote for ParknFly, but if you're using the cheapest lot during peak periods/times, you may have to wait a long time for a shuttle bus since they're capacity restricted. Buses for the more expensive ParknFly lots were more frequent.

I've had a lot of problems with the ParkNFly shuttles for other than the valet parking. They've often been infrequent and crowded. Once, the driver fell asleep on the Economy run. Don't expect much and hope to be lucky but at peak times it's a challenge. Direct Flight is the cheapest but they used vans that had serious mechanical problems and I gave up on them. Skyway and other small operators can be okay, especially if they remember to clean the van. The outdoor GTAA lot is convenient to the train and worth considering.

Bumped: paylessparking.ca is back with their coupon
 

Ths payless site is the way that ParkNFly charges less than others without revealing who it actually is. You simply print the coupon and then use the ParkNFly self-park lot. The coupon site was shut down during March break but is alive and well now.