If things go south, a verbal agreement to pay rent, utilities, etc isn't going to do you any good. Getting things in writing also gives you both something to go back and look at when there is a question, or someone isn't living up to expectations. This is even more important if both of you are not on the lease, otherwise you could be left with little practical recourse if things go poorly.

Approaching your potential roommate about a pre-nup is a task that will require you to put on your charm and avoid putting them on the defensive. Usually starting out saying that your goal is to protect both of you in the event of a problem will work well. Again, make sure you don't imply that your new roomie might "flake out"; you're just doing what lots of other roommates do. It also acts as a framework so you don't step on each other's toes later, and it will get you both talking about your expectations.

Creating Your Agreement
This section will not give you a template to edit and put in front of your roommate. The key is to work on your agreement together, use it as a basis for discussion on how things should be. Our goal here is you give you some good starting places, things to talk about, and include:

Sharing
Sharing is the key to making things work, and getting the details of what you're going to be sharing and how into writing is a good idea:

Rent - obviously you're going to be splitting rent, will it be 50/50? Is someone paying more for a master bedroom?

Bills - Are you both going to split the bills or take turns? Is there one bill (like the phone) that you agreed to pay more for because you'll be using it more? Setting up a spreadsheet tracking who's paid what is never a bad idea, and will let you both see that things are, in fact, as they should be.

Supplies - Every residence needs a constant supply of consumables. Be it cleaning products, trash bags, TP, or soap. Agree to joint shopping trips, where you both pick things out and split the tab. Sometimes, though, one of you will need to run to the store and get some things without the other. Setup a spreadsheet or even a piece of paper to track both of your purchases, and keep your receipts. This way nobody feels like they're paying more, since you can quickly see how much each of you has paid out. Be sure to not include stuff that isn't a shared commodity, however, such as your really expensive shampoo, etc.

Cleaning - Come up with a list of the shared things that will need to be cleaned, and when you want it clean. Think about common areas like the living room and kitchen. Do you both want to set a rule to clean up after use or is it OK to leave dishes in the sink, and pillows strewn everywhere until morning?

Some things will need to be cleaned on a regular basis: Work out a schedule of the regular cleaning tasks and how often: Dishes, Bathroom, Vacuuming/Sweeping, etc.

Food - Sharing food is probably the best way to save money with a roommate. It's a win-win; you eat out less and spend quality time together cooking and eating. If cooking and sharing isn't your thing, laying out the ground rules for things is a very good idea. Do you want to partition off the fridge? Set rules for leftovers … you don't want food to be wasted, but you don't want to be the only one putting food into it the fridge. Like household supplies, joint shopping trips are good here as well. Again we recommend setting up a tracking method (piece of paper, or spreadsheet) so if you run out for something without your roommate; you don't feel like you're paying more than your share.

Common Areas - Do you want to set a policy of "Leave it like you found it"? Or is letting the mess go for a day or two not a big deal? Talking and agreeing ahead of time on this will help a lot. Also discussing your morning and afternoon routines is crucial, maybe you want to come home to a clean kitchen, or you will need to use the bathroom at 7:30 weekdays, etc. You don't have to put it all in writing to use the agreement as a foundation for discussion.

Guests
It's a good idea to agree to some warning if people will be over, at the very least. This prevents you from being put into frustrating situations, or worse, heated "discussions" with your roommate in front of his or her friends.

Also talk about and agree if and how often boyfriend/girlfriend sleepovers are OK. Even if it's a fuzzy number like 3-4 times a week, it will give you both a basis for expectations.

Think about also putting down when your weekday/workday quiet time is, you'll need your sleep if you need to be at work or in class at 8am every weekday, etc.

Moving out
Writing down how much advanced notice is needed before one of you leaves is always a good call. It will help both of you out in the event of an unforeseen situation such as your roommate getting transferred to another area. They know they have to give you two months notice, etc. It also protects you both in the event that things don't work out; the person staying isn't left holding the bag (of bills).