Lots of good stuff in this wide-ranging interview with Richard Garfield, designer of Magic: The Gathering and other titles:
I usually encourage beginning designers to self publish, not with the eye to making an ongoing business, but to understand the market as a whole, and get a publisher to take you seriously.
Play games you aren’t naturally drawn toward, and find out why they are loved by their fans.
Play critically, but don’t lose your sense of fun.
Pay attention to the parts of a game people complain about – can the games be changed to make those people happy without taking the soul out?