One of the best ways to become an angel investor in early-stage private companies is to join a local angel group.

A very good article from David Rose (founder and Chair Emeritus of New York Angels) regarding the value that can come from joining a formal angel group.

The benefits of joining a [angel] group include pooling deal flow and capital, domain expertise, and investing experience. Most groups run regular education sessions for new members and provide mentoring for less-experienced investors.

Full Article on Marketwatch.