Some of these are already mentioned but:
If you have a large group, don't be afraid to split up. A few years ago we went with a group of 8 adults. The first day we hit MK early, three hours later we had only gone as far as Hall of presidents, and there were essentially no lines. At that point we all exchanged cell numbers and broke off into smaller groups. that way we could meet up here and there for a bit, but everyone still got to see what they wanted.
Plan on a midday break, even if staying in the park. This isn't only for those travelling with little ones. Catch a mid day show, go back to your resort for a couple of hours or find a shady place and people watch. Even though we are an all adult group, we still can have meltdowns when the heat and excessive walking get to be too much!
Ladies, use a drawstring backpack instead of a purse, that way your hubby can carry it.
I do this every year, and each year my best friend tells me she wishes she thought of that as she tries to carry her oversized purse.
Check parade times, even if you don't plan on doing parades! Trying to get through a section when everyone is lined up for the parade can be troublesome. My best friend and my hubby were trapped behind the Jungle Parade at AK, while her dh and I were having pizzafari without them.
If your group has split up, be specific when trying to set up a meeting place. A couple of years ago we agreed to meet at the carousell in MK. It took an hour before we finally called them, only to find out they were at the carousell of progress.