For starters, make sure you pick a companion that you are extremely compatible with. You don't have to be best pals, just be able to get along during the rigors of travel.
Meet in person and brainstorm over the details of the trip one or two times before embarking and have some good maps handy to help you plan. Take your timetable and your expenses into consideration and make sure you're both on the same page in terms of what you want to see crossing America.
Make sure you're signed up for an auto emergency service and try to have a reliable vehicle as well as a cell phone. Determine how you'll share the driving time and gas expenses before leaving.
Try to find accommodations ahead of time if possible and find ones that fit within both of your budgets. Camping can be a fun adventure and a good cost saver.
Bring some good guidebooks along to peruse during your travels. If you're only going to be in Chicago a day, for example, you'll want to find all the best things to see before you get there.
Find some ways to preserve the trip for posterity. Videos, pictures and/or a travel journal will be great fun to look at in 20 years.
