Knowing I wanted to found additional startups I always had my eye out for a great partner. I wasn't finding anyone I thought met all the criteria, yet I held fast to my commitment to not make the same mistake again. Ideas came and went but I was determined to not start by myself or with the wrong person.
Last summer I met a guy I could work with. We completed a detailed project and then decided to apply for Y-combinator. Nothing was done by either of us to facilitate finding this relationship - in fact it was almost by chance that we met at all - however it's clear after only 6 months of working together that it works. When it works it works.
My advice is to be patient. Never force the relationship. Network - but do it to meet new people not specifically to find a partner.
The only people who will react favourably to this kind of approach are the ones you definitely don't want to marry or start a business with.