Each member of your family should make a list of what they'll need for the trip, given the itinerary and activities you have planned. Don't forget the basic toiletries.
Inexperienced travellers usually pack TWICE as much as they need. Plan your wardrobe in layers, so that as temperatures rise or fall, you can add or remove an item. Plan your wardrobe around a limited colour scheme, to make mixing and matching easier. Wear colours that show dirt less (ie, khakis for beach vacations). Don't forget, in a pinch, you can wash clothes or shop along the way.
Pack bulky or heavy items at the bottom, and delicates at the top. Pack the frequently-needed items last. Put toiletries into a single bag, and transfer liquids into plastic bottles. Save space by stuffing shoes with socks or underwear.
Pack a special bag for special stops (a pool stop) so you don't have to unload the entire suitcase. If you are flying, pack your first day emergency toiletries and clothes into your carry-on baggage, so you won't have to panic if bags are lost or mis-routed.
Remove old labels from prior trips. Use tags with plastic or leather straps so they are firmly attached, and ensure your current address and phone number (or office number) are displayed in case the bags are lost and then found.
Load large suitcases first, and then load the smaller bags around them. Place garment bags flat on top.