Though the common sentiment here is not to start a company with a friend; I respect the general opinions in the article and comment threads, thought I must add that 'trust' is key. Typically those you trust are friends or family; unf. a Catch 22. I've started a few companies with friends and have learned a few things along the way. They are as follows:

1). Legitimize the business. Get the paper work done and a lawyer and advisor on board. The ability to have an unaffiliated third party arbitrator it key.

2). Definition of mutual goals. Know why all parties involved are even considering being a part of the venture, make it transparent and relate possible exits to that.

3). Define roles and respect them. If you co-founder is a designer trust his judgement; likely-hood is that he/she will do the same.

4). Define bailout plans. I you or your co-founder decides to bail; have a plan and make sure everyone involved knows of the plan. Hopefully the team 'trusts' each other to execute it incase they decided to seek greener pastures.

@lionhearted has some great points as well. Great thread and article.