Oh it is such a nightmare isn't it!? With Abby I just scoop her up and carry her under one arm, and then get the car seat in the other hand (only had to do that once though - but that's because we haven't been out much with Milly). I find that as soon as we are on our way she calms down very quick - It's like she thinks "oh well, that tantrum didn't work" and gives up.
Sometimes it helps to put a positive spin on things, like rather than saying "it's time to go" say something like "let's go home and get a buiscuit" or "lets see how many cars we can see in the car park" I also keep an emergency pack of chocolate buttons in my glove box - so I can use that as a bribe too. (Abby will do ANYTHING for choc choc!)
Also - this sounds obvious, but toddlers are more likely to have a meltdown when they are hungry, thirsty, or tired. Quite often we have come unstuck when Abby has been so happy playing that I have forgotten to offer her drinks and snacks, and it's not always easy to spot when she is getting tired - when she's so busy doing other things.
That's probably no help at all - but it is what works for us! Good Luck!
Nx