Make it a game: Challenge your toddler to a race to see who can get to the car first. (Then dare him to get buckled up just as fast.)
Save the best of your kids' CDs for car trips. If your toddler knows the car is the only place he can listen to his favorite tunes, he'll be more willing to get in without a fight.
Stash a few toys that haven't been played with in a while in the glove compartment as bait for especially whiny days.
Keep an extra bag in the car with a few necessities -- diapers, clothes, and a snack, say -- so you're not slowed down by lugging a heavy diaper bag.