You also want to fire people who a) create office politics, and b) who are persistently negative.
The second part of how to hire: try not to.
You're either not hiring at all or it's probably your single biggest block of time.
You really want to know your cofounders for a while, ideally years.
The most common post YC failure case for the companies we fund, is they're incredibly focussed during YC on their company... and after they start doing a lot of other things. They advise companies, they go to conferences, whatever.
You also really want to take the time to think about how the market is going to evolve.You need a market that's going to be big in 10 years.