We have gone once a year every year since before we were married, and we have three kids DS 8, DS 5, and DD2 - so we have gone with the kids at all ages - from infants up to where they are now. We love it and always have a great time!
I agree with what other posters have said, the best advice is don't think commando, you're not going to do everything and see everything. I also agree about splitting up sometimes, and changing up who rides with who - it's all great advice.
One other thing I wanted to point out in terms of the older ones, is that it's good to make a plan with them about "how we choose what to do". In our house it means they have to try and enjoy the fact that little sis still loves It's a Small World (like they did when they were her age) - and that enjoying your sisters happiness on a ride is as important to the family as them having their own vacation fun. We always look at scrapbooks of our past trips with them when they were little before we go to try and reinforce this. This has been very successful for us and they have embraced the concept completely.
The other thing to try and get them in the loop about is geography. WDW is large and there's always drama when one of them just HAS to go on a certain ride next, but they don't understand that it's a twenty minute walk away. If they will, go over the park maps with them in advance, ask them to point out things they are interested, try and give them some sense of the geography.
Make your own family's version of how the plan will go. In our family, we let them each pick the next ride or activity WITHIN THAT VICINITY one a time. For example, we're standing in Fantasyland and we say okay here are your choices, and we list them, and then they each get to pick one at a time. If they keep saying "I want space mountain" - we remind them that Space Mountain is in tomorrowland and that we WILL ride it, but not right now.
And the last most important thing to remember, others have said it but I will say it again, BE FLEXIBLE! It's the certain path to maximum enjoyment. If you miss your Crystal Palace ressie because someone's melting down halfway across the park - you will still find food and won't starve. If the little one falls asleep in the stroller right when the parade you waited an hour for is finally starting, than the rest of you can enjoy it.
We always joke about our WDW trips because I think we go on far fewer rides than alot of families with kids our age, but I think we have more fun! We certainly have less stress.
Pixie dust,
Susy