Fantasy is a beguiling genre. The allure for young writers is obvious: a fantasy world contains creatures, characters, and situations not found in nature. Fantasy may seem like a genre without boundaries or rules. This is misleading, because fantasy does have rules, which the writer is responsible for creating...then sticking to.
If there is one unbreakable rule of writing--in any genre--it is that you should abide by the laws of the universe in which your characters live. Once created, these laws should remain in operation unless you can break them in a way that follows logically from the story. Likewise, once you’ve shown how your magic works, you need to be consistent in the way you use it.
This article also served as the framework for an online writer's workshop I did for MuseOnline last year. This year I'll be doing one on being a professional amateur.