Do you write things down? Run yourself through your ideas a few times. If you see something you don't find satisfactory, you can make adjustments to it. It also helps to get some outside opinions, since they might notice something that you won't.
said as well, practice makes perfect. You don't need to start with something complex. Start with something simple. A fanfic would be a good start, since you can have some sort of foundation to start with.