First off, it is all practice. In my mind a good photographer is ALWAYS practicing. You may take the best picture ever, and you should be proud of it, but you should also always be thinking "how can I do better". It is all about building on your past mistakes and triumphs.
Secondly, look at what other images you like. Now, this is tricky, but don't try to reproduce them, but try to let them influence you. Look at cropping, angles, exposure, etc. and see what you like. Do you like contrasty images? Tightly cropped images? Images where the subject is off center? Look at what you like and try to work off of that. Everyone has different tastes.
Thirdly, post images here in the critique gallery. I have found that APUGers tend to be GREAT critics. They don't criticize, but critique. This can give you ideas to build on.