But in the event you don't have access to a technological solution try this: If you have an infant in your car get in the habit of taking your shoes off and putting them next to the baby when you drive. When you reach your destination you might forget the infant...but you won't forget your shoes.
Edit: My dad (who was an absent-minded engineer way before I was) taught me this trick.
Edit 2: Just called my dad to remind him about this and he said "it was only one shoe...the one you don't use on the pedal". I've been doing it wrong all this time :). So the correct way is...put your non-pedal shoe next to your child. You'll remember it when you start walking :)