When I was researching various things for our beach vacations this summer I ran across the single best tip ever. It sounded so bizarre, but then I tried it and it was magical.
If you're going to the beach, bring baby powder. Baby powder makes sand it's bitch.
There's not much you can do to keep half the beach from coming home in your car, but there is plenty you can do to remove all of that stubborn sand from your feet, your hands, and your legs. Not to mention that you will leave refreshed and smelling good instead of feeling like you're wildly uncomfortable because you're covered in grit and must find your way to the nearest shower.
Get a small bottle of baby powder and leave it in your car. Before getting in, put powder on everyone's legs and feet and when you rub it around the sand will fall off like magic. Sooooo nice.