If you want to end up with a clear and binding premarital agreement (also called a prenuptial agreement or "prenup"), you should get help from a good lawyer. In fact, you will need two lawyers -- one for each of you. But before your lawyers start drafting the prenup, you and your loved one should decide on the essential terms of the agreement. (To decide if you want or need a prenup, read Nolo's article Is a Prenuptial Agreement Right for You?)
There are good reasons why seeking legal advice when making a prenup is advantageous. In fact, each party to the prenup should get help from a different lawyer. Here's why.
The laws governing marriage contracts vary tremendously from state to state. You can certainly do some of your own research to find out general information on your state's laws relating to prenups. (Nolo's book, Prenuptial Agreements: How to Write a Fair & Lasting Contract, by Katherine E. Stoner and Shae Irving, can get you started; it provides information about each state's laws, plus legal research tips.)
But, if you don't want to invest your time learning the ins and outs of your state's matrimonial laws, a lawyer who knows the intricacies of those laws will be an important resource. The lawyer can help you put together an agreement that meets state requirements and says what you want it to say.
This explains the desirability of having one lawyer, but why two? Prenuptial agreements are still scrutinized by the courts, sometimes very closely. If you want your agreement to pass muster, having independent lawyers advise each of you can be critical. While most courts don't require that each party to a prenup have a lawyer, the absence of separate independent advice for each party is always a red flag to a judge.
On a practical note, having separate legal advisers can help you and your fiancé craft a lasting agreement that you both understand and that doesn't leave either of you feeling that you've been taken advantage of.
That said, it's best not to ask your lawyers to start writing up a draft or final agreement until the two of you have settled on its essential terms. You should put those terms in writing -- either in a written outline, or a draft agreement you create yourselves. A prenup prepared by a lawyer who isn't working from terms you've both agreed on is likely to be one-sided and adversarial. If you provide your lawyers with an outline or draft prepared by both of you, the whole process -- and the final document -- will be more balanced.
All of the above assumes that you select and use lawyers who are not only competent and experienced in matrimonial law, but who are also capable of supporting the two of you in negotiating and writing up a loving, clear, and fair agreement. Finding the right lawyers can take some time, but it's worth the effort.
For tips on finding a lawyer, see How to Find an Excellent Lawyer. You may also want to visit Nolo's Lawyer Directory. It provides a comprehensive profile for each lawyer listed, including the lawyer's experience, education, and fees, and perhaps most importantly, the lawyer's general philosophy of practicing law. Nolo has confirmed that every listed attorney has a valid license and is in good standing with their bar association.
You can use Nolo's book, Prenuptial Agreements: How to Write a Fair & Lasting Contract, by Katherine E. Stoner and Shae Irving, to draft your own prenup. The book thoroughly explains what you can and can't do with a prenup, helps you decide whether or not you need one, and walks you step by step through the process of drafting your own agreement. It also provides detailed information on every state's laws about prenups and marital property.
For same-sex married couples and registered domestic partners in California, there's also a companion eGuide, Prenups for Partners: Essential Agreements for California Domestic Partners, by Katherine E. Stoner (Nolo).