Just a few--others may weigh in with more, even better tips!
1. Money: I get travelers checks in advance (US) to use for the guide tips at the end of the trip--I feel better carrying those around than cash. I use the tip info provided in the booklet
ABD provides, and usually get more than the max to have additional "emergency" money. Otherwise, I get enough of the local currency to use at airports, and for small items--and for additional cash use ATM machines (although with fees, try to only use it once if you can). Best to use credit cards whenever you can.
2. On one trip I ran out of socks--don't know how that happened! Took time away from touring around Venice looking for a store that sold socks; found one and paid far too much. After that, we found shirts/shorts/underware that was lightweight and could be washed in the sink and worn again if needed.
3. Converters/adapters: if you are bringing any items with rechargeable batteries, be sure to bring the charger and an adapter! (Or plenty of extra batteries if not rechargeable). Our our first trip with Disney we were in Rome for an extra day before the trip was to start, and while sightseeing the humidity did something to my camera. Yikes!! I ended up buying a camera at the Rome train station (a large electronics shop is there)--and how have a european charger that I have to use a US adaptor plug with at home to charge the camera battery. But you have to have a camera! (On that note--I now bring an extra camera card, extra batteries, and a backup disposable camera just in case--and the Disney guides take great photos which you will be able to access on-line after you are home from the trip; we used these photos to make an album which I really really like).
4. Most important of all: you are paying the big bucks for ABD because they take care of you from start to finish. If there is something missing or amiss, just ask one of your guides! So the biggest tip of all: relax and enjoy the ride, and the sights for an adventure of a lifetime!