The other component of being a good angel investor is simply to be a good person. Angel investing is not a business where you make money by screwing people over. Startups create wealth, and creating wealth is not a zero sum game. No one has to lose for you to win. In fact, if you mistreat the founders you invest in, they’ll just get demoralized and the company will do worse. Plus your referrals will dry up. So I recommend being good.
The most successful angel investors I know are all basically good people. Once they invest in a company, all they want to do is help it. And they’ll help people they haven’t invested in too. When they do favors they don’t seem to keep track of them. It’s too much overhead. They just try to help everyone, and assume good things will flow back to them somehow. Empirically that seems to work.
The essay by Graham that follows this one on being relentlessly resourceful to be a successful start-up entrepreneur is very good too. Have fun!