Portrait photography can be a tricky genre, as it relies very heavily on your ability to interact with and bring the best out of the people in front of your lens, making it just as much psychology as photography. This great video will give you some tips for working with subjects as well as some helpful career advice.
Coming to you from Marc Silber at Advancing Your Photography and Jessica Sterling, this interview covers a wide range of portrait photography topics. Of them, I think Sterling's career advice about judging one's own work is particularly salient. It's important to always remember that photography is a business with a product, and as such, the market is the judge of that product's economic viability, which should be divorced from whatever internal judgments you hold about its artistic merit. Of course, that doesn't mean one should ever stop objectively asking themselves what they can do to improve their output, but I think Sterling's point is more about having the confidence to present your work as worthwhile and a product worth buying, as confidence is absolutely crucial, particularly in a genre like portraiture. The confidence you exude will rub off on your clients when they're in front of your lens.