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IN MANY WAYS, ZANE Carter, 56, and Ken Cosgrove, 55, are your average couple. Together for 24 years, they've seen their fair share of ups and downs. They've even gone through counseling.

But Carter and Cosgrove aren't married. In fact, they've never even dated. They're business partners. Together they run Alexandria, Va.-based Carter Cosgrove and Co., a design firm launched in 1981 that now employs 19 people.

Ask them about the key to a successful partnership, and they'll tell you it takes a lot of work — and patience. "It's very much like a marriage, though I get along better with my wife," jokes Carter.

Still, as business partnerships go, it has been pretty smooth sailing for the former college buddies, who chalk up their success to having complementary skills and temperaments. Sure, they've had their rough patches. Like back in 1983, when an argument over something now long forgotten inspired them — on the advice of a psychiatrist — to sign up for a "bipolar training weekend" sponsored by a local church. But over the long haul they've had "a pretty damn good time," says Carter. "If it's not fun, it's not worth it."

For small-business owners, partnerships have lots of appeal. A partner can share the financial burden, listen to ideas and generate them, too. And a partner can watch the shop when you take a much-needed vacation.

Even so, partnerships aren't for everyone. Entrepreneurs, by their very nature, tend to be independent types. So if you're the kind of person who doesn't play well with others — namely, you don't share space, money, or credit well — you're probably wise not to waste your time looking for a sidekick.

But if you think two heads might be better than one for your business, here are some tips on how to create a healthy partnership.

1. Fish For Complements
You might be tempted to look for a business partner who's a lot like yourself. Employers do it all the time when they hire employees, says Kenneth Siegel, a psychologist and president of The Impact Group, a Los Angeles-based consulting firm. They're looking for people they're "comfortable with," and that often means a person who's a lot like them.

But it might make more sense to look for a business partner whose skills and background are quite different from — but complementary to — your own. Ideally, "you're selecting a mate who brings to the party that which you don't have," says Siegel. If you're too similar, you might get along superbly in the beginning, but you'll end up in a power struggle later on.

That said, you don't want to be polar opposites. You should share some core values, says Alexander Moss, a principal at the Praxis Consulting Group, a Philadelphia-based organizational consultancy. While a "diversity of styles, habits, and backgrounds is essential," he says, you must have similar views on ethical issues as well as some practical ones, like the long-term financial goals of the business. For example, do you see your business as a fast-growing profit-maker or simply a way to make a living? Will information about the company's financial and other dealings be closely held, or widely known? What kinds of things would constitute a breach of trust?

2. Date First
Before you sign on the dotted line, be sure to get to know each other first. The marriage analogy is, again, apt. You might feel you know your potential business partner well enough, but have you spent time with him or her in both professional and personal settings?

Before you make a serious commitment, play golf together, go out for drinks — take steps to get to know each other, says Paul Edwards, an executive coach and the author of "Teaming Up: The Small Business Guide to Collaborating with Others to Boost Your Earnings and Expand Your Horizons." Listen to the stories the person tells when you're not talking shop. Does he or she seem honest and straightforward, willing to compromise and communicate? If possible, try collaborating on a not-so-serious project first, he says, such as wallpapering a room or planning a party together.

If that goes well, try a more serious joint venture. If you both run businesses, stage a cross-promotional event. If not, try to pull off a long project — a joint proposal, a co-authored report, a collaborative marketing campaign. It's a good test to see whether working together on a daily basis will be feasible.

If all of this goes well, then it's time for some due diligence. Ask your potential partner if you can talk with his or her former colleagues and employers, and offer the same of yourself. Ask if you can check credit and criminal records, and, again, let him know he can check yours. This must be done delicately, of course, but no matter how well you think you know the person, it's critical, says Edwards. Entering into a business partnership can mean that you're liable for nearly everything your partner does — from failing to pay bills to causing an auto accident when he's running a business-related errand — so you owe it to yourself to check things out thoroughly. 3. Put Everything in Writing
Once you agree to form a partnership, put all financial and decision-making details on paper.

In the early stages, this is something you can do on your own rather than in your lawyer's office. Sorting out the finer points may take weeks or months, and it's important that you and your partner work them out together, rather than let a lawyer figure them out for you, says Denis Clifford, himself an attorney and the author of Nolo's "The Partnership Book."

Some questions to consider: Will ownership of the company be split 50/50? Will profits be split equally — or will each partner be paid according to how much he or she contributes? What constitutes value in your business? For example, is rounding up new clients just as valuable as managing employees or keeping the books?

Be sure to discuss how decisions will be made, says Clifford. If you and your partner disagree about a major strategy move, who will break the tie? Is it okay for spouses, other employees, or advisers like board members to be involved? Set written timelines for seeking outside help.

And although it might seem pessimistic, you must also consider all possible scenarios surrounding the breakup of the partnership. What happens if one person leaves the business? Can a partner's stake be sold? If the business fails, how will the debt be split?

Once you've hashed through all these details, you or your lawyer should draw up a contract. A written partnership agreement is legal once it's signed. If you're comfortable with the terms and confident that you covered everything, there's no need to take it to an attorney, says Clifford. If, however, there's a good deal of money involved or the contract contains complicated clauses, you should let a lawyer review it before signing.

4. Renegotiate Often
Once you've hashed out the details of your partnership, be ready for them to change.

Over time, one partner might feel the terms of the relationship have shifted — he's contributing more than his fair share of the work and deserves more of the profits, or his family situation has changed and he's no longer able to invest what he once could. Whatever the reason for the shift, says Clifford, be prepared to hear your partner out. "You might have the same contract for years, and then renegotiate it three times in one week," he says. No matter the frequency of the talks, it's important to roll with the punches.

In fact, it might be healthier to see a business partnership as a tentative situation, one that works only as long as the collaboration is functioning well, says Siegel of The Impact Group. Along with frequent, small renegotiations, partners should write time limits into their contracts and plan for bigger renegotiations every five to seven years.

Viewing a contract as tentative can be liberating. "Seeing the partnership as an opportunity instead of as an obligation — it's a powerful way to prevent a breakup," Siegel says.