Orson Scott Card wrote a whole book, titled How to Write Sci-Fi and Fantasy.
Really though, the best way to get better is to simply practise. All the how-to manuals in the world are no substitute for practise. Approach like you would most other genres. Don't use purple prose, but don't go beige prose either, careful with infodumping, and don't overpower your characters.
If your characters have special powers, remember them, because your readers will as well. For example, if your character can turn into a mouse, well you can't really trap them in a convential cell, otherwise they could change and leave at their own will. Don't have a flying character die from a fall, unless you did something specific to disable their power (such as hurt their wings, or something mystical if they just float).