From personal experience, you just have to keep trying. Make a blank 500 x 500 sheet, and just start messing around with the tools. Think of colors that would appeal to the eye and then work with those. Try simple things, and then combine those things to recreate what you have in your head. It's not always easy -- I have a wild imagination with every single detail in mind, but I can't translate it into Photoshop/Flash (I usually come up with cartoon/animation characters) to save my life!

Good luck!