The biggest thing to forget to do is reventative maintenance! Do everything possible to not ask for that breakdown deep in the unknown. Spend a day or two getting to know your steed better than you already do. Carry tools to do all regular maintenance......you'll be surprised when something stupid raises it's ugly head! Credit cars, and duplicate paperwork. But also most important a spare set of keys! No I haven't had the htrill of using them yet....but seeing as they are hidden well on my bike at all time.....murphy stays away for that possibility..
I agree with do not over plan your ride....simple day to day plans that can change with the wind are best....and take enough fundage capabilities to get home. Health ins. if that is a concern for below the border, I'm above so we don't have that issue as much from what I hear, but I also have extra just to be sure.
OH....the biggest problem is who you choose to ride with.....it could mean the end of friendships or even a ruined vacation. I have suffered both.......it's very surprising what will get under your skin after a while on the road.