Decide upfront why you are going and what you want to get out of the trip.
Money has the potential of destroying the best of friendships, let alone create problems if your travel companion is a mere acquaintance. Its best to decide what, where, and how much you will be spending on mutual expenses.
Prepare a tentative itinerary; places you want to visit, things to do and time to be spent at each location.
Travelling with some one has benefits, but even then, you should get sometime to yourself. Make allowance for time that each will be spending on their own.
Every one enjoys different aspects of travel. It s good to figure out each others preferences before getting stuck together on a rather costly trip. Some of us are night people while others can t keep their eyes open after dark. Some like everything decided and planned, others are more spontaneous. Some like to indulge in the culture while others prefer to immerse in the sights only. Make sure you and your companion share some views, if not all, regarding travelling.
Unless you are a 100% sure about the person you are travelling with, pack some earplugs, they can be a life line during an otherwise sleepless night.
Have an open mind & a generous heart. Give the other person some space & make the most of your solo time as well. Have fun and keep it enjoyable for both.