Pretty much what everyone has said here is good sound advice. Here is my opinion on the matter.
1. It all boils down to finding out what you need to learn in order to get a good design sense.
2. Then tracking down good books about design.
3. Next track down good online resources about design to supplement your knowledge.
4. Watch what the best do and learn form them.
5. Then it is all about experimenting and trial/error at this point.
Practice alone won't get you good at design, perfect practice will though. Perfect practice is basically practicing with a lot of concentration and focus so you pick up on things you never seen before and continually learn from your experience rather than just do A, B, and C over and over again without thinking or reflecting on it. Good luck, it is a very rewarding experience.