I have been going to WDW since I was 7, now at 25, trips are way more laid back and relaxing. Things that we have learned over the years are as follows:
When traveling in a group..people get crabby...even if its no one's fault. Everyone needs a little space and alone time. Some time just splitting up for a swim break or a nap or a walk around the resort is a quick fix for crabby people. Especially whe you travel in a group of 6 or more with combined familes..my family, my sisters family and inlaws and some friends..lol
Not everyone is on the same page when it comes to morning or night schedules. I my self am a night person. I work 2nd and 3rd shift jobs. I'm a EMT wich has me up all hours of the night. I like to sleep till noon if I can. On vacation I don't sleep the day away but I also don't like getting up before at least 8am. SO We plan our days around each other. My mom usually gets up around 7 very quitely and goes for her morning walk and then gets coffee. Im usually up or getting up when she gets back and then we get going.
We've also learned it is worth the extra money to get seperate rooms when traveling with a group of 4 or more. We usually travel with my mom, my self, my sister and her fam (BIL and NIECE) and its hard to have all of us in one room, even a big room. They need space for Keegan and getting her ready and putting her to bed. My mom and I like to have a drink or watch tv before bed and I like make some phone calls to friends and so on and when we would share a room we couldn't do that till after we had the baby in bed. Also its hard with only ONE bathroom.
Also a mistake I see people make and that we used to make was being RUSHED. WDW is huge and there is a lot to do, but if you don't take a second to take it all in and relax you're going to need a vacation from your vacation! Plan out your days and know what the park hours are and where and when your ADR's are so that you can try to have time to do what you want and still enjoy some pool time or even a half hour to sit in a hammock.
I guess what I can say for mistakes I personally make is getting irriated with the people I'm traveling with. Wich is why I stated above.. that alone time can do wonders. We are on vacation at THE HAPPIEST PLACE ON EARTH.. so try to stay calm and pack your patience.. or as the BIL say's drink a mood adjuster.. (MMMM RUM!
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Learn from your past mistakes and everyone's that have shared.. and have fun!!!!! Don't "sweat" the small stuff..
SPEAKING OF SWEAT.. a mistake we USED to make but shortly leared after say our 3 trip.. over pack a bit so you can put on FRESH clothes after an afternoon swim and shower! Now we just do laundry halfway through the trip.. and always check the weather and plan accordingly.. Were going in Jan so i'll have a bit of everything just incase..won't make that mistake agian.. We went in Dec back in say 98 and BRRRR..Poor dad spent over 200$ getting all of us sweatshirts and hats.
Oh and last but not least..if you have questions ASK.. there are always CM's around to answer and if they don't know the answer they can help find some one that does!! It will save you time and frustration. Beeing a seasoned Dis Vet. I've even stepped in and helped a few "lost souls", they just had that look on their faces so I knew they needed some guiding!