I've founded three companies and am in the process of founding a fourth. Out of the first three, the first company had one cofounder, the second had three (too many!), the third and current one is just me, and the new one will have two.

My first two experiences with cofounders didn't work out so well, with one exception. This time around, I'm putting to use what I learned from those first two experiences and the approach seems to be working.

A few specific lessons I've learned the hard way -

1 - Just because someone is a friend doesn't mean they'll make a good cofounder.

2 - Spend time getting to know a person's personality, specifically what their values are and what motivates them. While you don't have to and shouldn't agree on everything, make sure your overall values and motivations are in alignment.

3 - Be honest w/yourself about your own strengths and limitations and find a cofounder who complements your limitations and is open to benefitting from your strengths, by being aware of their own strengths and limitations.

4 - DON'T rush to make a decision. Taking the extra time to find the right cofounder will save you time in the long run. I've made the mistake of finding someone I thought might be a good fit, just to get a project moving, rather than finding someone I was convinced was a good fit.

5 - Look for past successes and/or someone who is honest about past failures. Regarding failures, look for someone who can look at them objectively, while also acknowledging his/her own role in the failure and what they've learned from it.

6 - Don't make the decision late at night over a bottle of scotch, even if it's really good scotch :)

Best of luck, always happy to discuss further on a call as well...!