1. What does everyone want?
This is by FAR the most important thing, and important to discuss before any further planning. Just because you all want to go to Greece doesn’t mean you all want the same thing out of it. Sure you might say “oh the beaches” and “ooo, ruins” but if you’re thinking 90% beaches and 10% ruins, and they’re thinking the opposite, there’s going to be conflict.
Just because you want different things (and of course you will) doesn’t mean you should cancel, but we’ll get to that later.
2. If you can be flexible, do
As with any relationship, there are going to have to be compromises. The problem when traveling with friends is that the mechanism for compromise isn’t as ingrained as it is with a spouse (presumably). Also, somebody is going to be a little more of a finicky eater, a little more squeamish about accommodations, a little more worried about money.
So if you can compromise, do. Stand ground when you need to, but being more like water and going with the flow will make everything go smoother. This is probably the single biggest change for me that made my travels with friends better.
3. Don’t focus on pennies
Money is the cause of more arguments among friends than probably anything else. If you let yourself fixate on a few dollars here and there, it’s going to ruin your trip, and if it blows up into something, risk everyone’s trip. Unless someone is obviously and knowingly skimming or cheating… well wait, why are you traveling with that person to begin with?
4. Figure out how to split meals before you go
It’s probably best to establish what the money plan is ahead of time. Chances are someone makes less than everyone else and is not looking forward to splitting a $400 meal 6 ways when all they had was pasta.
Discussing it ahead of time also gives an embarrassed party to talk about it separately from the group.
5. Don’t make ANY decisions when hungry or stressed
This is probably the most important rule. It always amazing how few people realize that everyone gets SUPER cranky when they’re hungry. It’s called "hangry," and holy Cthulhu does it lead to arguments. How do married couples not notice this?
Add the stress of transit to the mix (getting to an airport, missing a train etc) and together these are bad times to have a serious conversation. Let the cranky be cranky. Get them food. THEN figure out what the issue is.
6. As in all things, communication is key
A group dynamic is a fickle, delicate thing. Don’t let things fester. Don’t let things build up. A presumed slight, a mistake on a check, any number of things can seem like an issue but digging deeper, aren’t. Guys especially aren’t usually accustomed to talking frankly to their mates. A calm but awkward discussion is ALWAYS better than a fiery argument.