If you plan to sell your photos on microstock web sites such as Fotolia and istock. You need to make sure you have a model release for your subjects. Without a release, the publisher or agency can refuse to publish it, and if your model finds out you used his image unreleased, he can take legal action.
This model release form comes in pads of 50, is written in standard but easy to understand legalese, and has signing areas for both adult models and legal guardians of child models. Keep a pad of releases in your camera bag!