One 29" pullman and two 20" backpack carryons, plus a fanny-pack camera bag. Also the necessary carseats and a stroller when we used one; those were all gate-checked. That's for 2 weeks in Europe. For WDW we go lighter.
The large case can end up going overweight (50 or 70#, depending on your airline and your route), so I also put an empty duffel on top in the pullman. I pack using space bags, so if we go overweight, I just pop a couple of the bags into the duffel and check that, too (it already has a luggage tag, of course). When we arrive at the destination, I re-consolidate so that I don't have to drag bags and children.
We pack the same way for road trips as for flying. I once caught myself overloading the car for a road trip, and vowed that if we didn't need it flying, we didn't need it driving, either. (The only exception to that is pillows.)
Two good tricks I know for avoiding overpacking: never pack directly from the closet into the suitcases; lay it all out on a flat surface first, so that you can see just how much stuff there really is. Once you've done that; take a hard look and try to put half of the stuff back in the closet before you start loading the suitcases.
If you forget a piece of clothing, you can always buy. Bring a little woolite in case you need to wash undies in the sink.
Oh, one more tip. NEVER segregate clothes into separate cases for each person. Always mix it up. That way, if one bag is lost, that person will still have something to wear. Carry a spare shirt and a change of undies for each person in a carryon.