I had to do that a few times and I felt terrible, too. Fortunately this phase doesn't last long.
Because its so hard for them to choose to get strapped into that terrible carseat, I think its better to go ahead and do things you might not ever use under any other circumstances - treats.
I kept a stash of special treats in the car that I would give to DD after she was in the seat, saying something like, "I know its hard for you to be in this seat. Is there anything you would like that might help make it easier for you? How about a fruit leather?" If she said something like "hot cocoa" I'd do that (go to the drive through coffee store for cocoa). I figure if I have to do something enormously unpleasant, I try to make it easier on myself, too. I'm not bribing myself, I'm being kind to myself, I think, like getting a vanilla latte for having to get on the road in the early morning or for a long drive, and a nice music CD.
I didn't use it as a reward (e.g. "if you get in the seat you can have X") I just gave it to her regardless of whether it was a fight or not. I also turned the seat forward facing as soon as I could, and had special car toys. After a while she allowed me to put her into the seat without protest (although it was a lot longer before she actually climbed into the seat herself). I was just happy to not have the protest anymore.
Incidentally, if you are worried about her expecting a treat every time she gets into the carseat, that doesn't seem to have happened as DD got older. Now she has the capacity to know where we are going and often that is motivation enough ("we're going to the park and then to the store").