First, I'd like to disagree with those who say business and friendships are a bad mix. Going into business with a friend (spouse, family member, etc) can create a very strong business which is able to weather hard times. Ever heard of Hewlett Packard? Harley Davidson? Electronic Theatre Controls (ETC)? Some of the biggest names in business started out as a couple friends in a garage.
The key is to not let the business replace the friendship. And don't assume that friendship is a replacement for good business practices.
Before you get involved in the actual business, sit down and talk about who will be doing what. Lay out clear guidelines as to who's responsible for what aspects of the business, and what to do when the balance tips significantly.
If there are only two of you in the partnership, decide who's in charge. Then give the other one 51% of the company. It's a balance of power.
Split the profits (and debt) equally. If workloads become unbalanced, shift responsibilities to make them balance out.
Before you do anything else, write up a document stating what to do in case of conflict. What to do if one partner wants out. What to do if one partner is acting in a manner detrimental to the company. Make sure that you both agree to it and that any spouses agree to it.
When conflicts arise (and they will), remember that this person is your friend.
3 years ago, I started a company with 2 friends. On the first of this year one of them left the company to spend more time with his family. We're all still friends—despite several major conflicts, including a coup d'etat where I took over the Presidency. The best thing we ever did was to write out the "buyout" rules before we started.