1. Take a good look
When shooting portraits, if you can choose a face, go for one with symmetry, and good eyes and bone structure. Learn to look properly at faces, as it's not always immediately obvious who's photogenic.
2. Think about the skin tone
The darker the skin, the flatter the light you need. If your subject's got bad skin, use a bigger, softer light source.
3. Make your subject feel comfortable
They want flattering shots. Study how light can slim down a face. Look at portraits you like and try and replicate the photographer's style. You'll then start to develop your own style.
4. Go for the right lens
Use a portrait lens if you can - it's nearly always more flatting.