My #1 tip: Don't over-plan. Keep it simple and fun!
#2-Keep it simple and fun!
By those, I mean don't try to micromanage every minute or every detail of the trip. No visit ever goes 100% the way we think it will, and the biggest stressor I see is when people get upset the trip isn't going the way they envisioned it going. If something isn't going according to plan- roll with it and modify!
For a first trip, I wouldn't plan many table service meals (less than 1 per day), or any other of the pre-paid activities WDW offers. (or did in the before times.) When looking to book a table service meal, if you are staying onsite, consider booking the ones offered at your resort. Otherwise, getting around WDW on time can be stressful. WDW transportation and weather can be a bit unpredictable.
Another - what I would call mistake- I think many first timers sometimes make is to go overboard with matching outfits or costumes and trying to have everyone look a certain way. Too often I have seen miserable families, because the items are uncomfortable or a family member is rebelling, and the family planner is yelling at everyone because it isn't going according to the plan they envisioned.
I can't keep it to just those, but most of the following also mostly = stay flexible:
#3 Do try to arrive early AND/OR stay late- BUT - only as much as it doesn't make your family miserable. If someone is tired/stressed, take a break.
#4 Break the rules a little! One of our secret little trip solutions is to eat ice cream for lunch, or at least it before lunch. DO NOT try to eat lunch at noon. Again, flexible! Eat a cookie for breakfast! whatever is easy.
#4- Zig when everyone else zags. Don't turn your brain off. Again, don't eat lunch at noon when everyone else is trying to eat lunch. Along the same lines, try to avoid ALL of the WDW rush hours if possible, like avoid starting your trip on a Friday night. For attractions, you do NOT need to research every nook of the parks, just know the 3 most popular attractions in each park. Do those early, late, maybe skip. (MK=Mtns + PPan. HS=Rise, Slinky, and M Falcon. Epcot = TT, FEA, Soarin', and will be Ratatouille when it opens. AK = Pandora+ Safari. US = H Potter rides). Also, WDW is SO much more than just the attractions. Your favorite part might just be a quiet garden or water fountain! Just go with it!
Mid-day, stick mostly with short(er) line attractions - via the app - if possible. Most attraction lines ebb and flow. If a line is long, walk away/skip it. Maybe come back later. Unless someone in your group has very limited endurance.
#5 STAY FLEXIBLE! As much as possible do NOT promise anyone that any specific thing will happen, or require any specific thing to happen. Attractions do go down, someone might get too tired, etc. Adults often throw the worst temper tantrums of all!
#6- Get in shape/be comfortable! Go for daily walks before you go, buy the most comfortable new shoes you can find! Highly consider the type of sandals that are water-tolerant + have robust support (Teva is one brand) AND a pair of high quality sneakers for everyone. Also in this category- minimize what you carry, and maybe even what you pack. Again, think comfortable. Maybe get a new roller bag if it is easier. I also love ziploc bags for WDW trips; many ways they come in handy. Along the same lines, I'm not sure how to say, but sometimes paying for an little extra service can make the day- like paying for transportation, or an upgrade of some kind: maybe that stupid fancy drink/stuffed animal. Some smiles are free, others cost $10. A small slush fund at WDW is a good idea for those unexpected little treats.
#7- Stay hydrated! One place you must be rigid is to make sure everyone drinks LOTS of liquids (except alcohol and soda), even if it means more bathroom breaks, and paying for them. Apple juice and lemonade are great, even watered down.