Plan a trip with another family and plan to stay together all the time. It was terrible. We had 5 people including ds8 and dd5. They had 6 people including children 2, 4, 5 and 8. We are up and out early people, they are not. Cousin's dh wanted to spend ALL of the time at the MK (9 days?). Halfway through the trip, my adult dd went off on my cousin's husband in front of hundreds of people at the bus stop at the MK. After that, we separated and met for dinner only and we had a much better trip (although my cousin's family missed a lot of stuff because it was their first trip and they had done no research).
Stay off site. I hate it. I hated the 16+ hour trip there, the drive back and forth, parking and trams - we always use Disney buses. I did love the villa and the extra space came in handy when 3 people got the stomach bug the same night. Although it was a direct route to the main entrance, I still got lost several times.
Plan an ADR the first night for 8 pm when we've been up since 3:00 am. Dd ended breaking a glass and spilling soda all over Mickey at Chef Mickeys. Not a good start to the trip.
Surprising the kids. Last trip was a surprise. Ds10 had the reaction we expected. Dd8 did not. She cried for 3 days - thought we were lying (didn't tell then until we arrived at the airport), didn't get to tell her friends and teacher goodbye, missed her Halloween party at school...it went on and on. Now they will know as soon as we do.
I'm sure there's more...these are the ones that came to mind first.