The agreement doesn't need to be complicated. In fact, the simpler the better. Make sure it's clear that both people will leave with all the personal property they brought, and specify who gets the house, the dog, etc. If you have lots of complicating factors—kids, large debts or assets—make separate contracts for each issue, rather than trying to create one giant document. And if anything is confusing, get help from a lawyer.
While decisions themselves may be complex, composing the document is easy. You can find forms online, but there's no harm in drawing one up yourself. Nothing is iron-clad, but most small claims courts will uphold any document they believe was signed in good faith—even if it's crayon on a placemat.
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But whatever issue the agreement hashes out or how it's composed, don't mention—or even allude to—sex. Courts get touchy when you use language that might be seen as legitimizing prostitution, and if your contract states that you pay the electric bill and he flips your switch a minimum of three times a week, a judge is likely to toss the whole thing out. One couple had their case tossed out because they referred to each other as “lovers.” It's best to keep non-monetary agreements—how many nights his drunk best friend can crash on your couch, who scrubs the toilet—out of it as well.
Unfortunately, there's bad news for residents of Illinois, Georgia and Louisiana: In those states, living together is still considered “immoral” and because of that, a couple can't create a contract defining the terms. Things may be getting better in Illinois, though, since one court decided that contracts that don't resemble marriage claims and aren't based solely on living together can be enforced. So if you're trying to do this in The Prairie State, see a lawyer. If you're trying to do this in Georgia or Louisiana, well, you might as well give it a go, since laws could change before it becomes an issue, but know that the document may not hold up in court.
A word about common-law marriages
Most states have abolished common law marriage, and even in the ones that have statutes (which, at the time of writing are Alabama, Colorado, the District of Columbia, Iowa, Kansas, Minnesota, New Hampshire, Oklahoma, Rhode Island, South Carolina, Texas and Utah), simply living together for an extended period of time isn't enough to get you declared husband and wife. In most cases, you have “behave” as a married couple, using the same last name, for instance.
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Doom and gloom aside, plenty of couples make it (and FYI, should you decide to tie the knot, your cohabitation agreement becomes void, unless you signed it shortly before marrying), so all your efforts may be for naught. So look at this as the first of the tough conversations you'll have as live-in partners. And after you negotiate this, that cap-off-the-toothpaste thing will be a breeze.