If traveling with a group delegate tasks. When I travel with my sister I put her in charge of flights. I tend to plan everything but we each have 1 or 2 restaurants we request and a few attractions we ask for fastpasses. This can be difficult if your like me and like to be in control. If you don’t delegate you may stress yourself out and not enjoy your trip.
Look at crowd calendars and know how busy the parks traditionally are. This is major!!! Since fastpass+ started you really need to plan. Know when the holiday parties are if your going during Mickey’s Not So Scary Halloween Party or Mickey’s Very Merry Christmas Party you need to remember that the Magic Kingdom does close at 7pm party nights and guests who have a party ticket will be coming in starting at 4pm. Be aware of special events they may have added crowds.
If traveling with little ones allow down time, either slow quiet rides to rest such as Hall of Presidents or Ellen's universe of energy or plan to head back to your resort. it will make for long days and cranky children if you think the excitement will keep them going all days. Even kids around 8 or 9 benefit from down time.
Allow enough travel time from your resort to parks for meal reservations. This is another super important one! At least once a trip I see groups of people arguing about this or I see people frantically trying to get to their restaurant in time. This happens to most people at least once a trip and can put a damper on your day. This happened to me during last year's birthday trip I was seething because we were running late for Be Our Guest because of transportation issues and my sister taking her time. Luckily they accommodated us since the hostess saw me about to cry, YES I was that mad/upset since I had been waiting to eat here since it opened. Don't assume that the restaurant will seat you if your late, during crowded seasons they may not be able to seat you if your late. Most people say give an hour to travel from your resort to the parks, I would say the same or even a bit longer if your a planner like me. Some restaurants or shows take longer to get to such as Hoop Dee Doo I would suggest 2 hours because you need to take a bus or monorail to the Magic Kingdom then a boat to Fort Wilderness. You could also take a bus to another park then board the Fort Wilderness bus but the boats dock closer to Hoop Dee Doo.
Pack sunscreen and band-aids. You don’t want to be miserable on your trip because you’ve gotten sunburned or end up in the hospital with sunstroke. Drink lots of water, the Florida sun is hot and the humidity can be killer. I always carry sunscreen, band-aids, blister block, and water while traveling during the hot season. During the winter I usually apply sunscreen at the resort and opt for a smaller bag unless it’s super warm then I’ll bring sunscreen to reapply.