Hey dagger19,
Improvements can come from a lot of sources, depending on what your weaknesses were. For example, if your weakness was lack of aviation knowledge, then reading more about aviation and joining some aviation-oriented activities such as the AAEP would be ideal. If your weakness was English, then joining some English courses would be good. If you failed the eye-hand coordination exercises then taking driving lessons could be the solution. The most important thing is to identify why you failed, and work hard at improving yourself. If you really have worked hard, HR will notice your efforts when you re-apply; just be sure to indicate your efforts in your application form/CV.
I'm glad I applied after 2 years instead of the published cool-off period of 12 months; I wouldn't have had enough time to improve had I applied that soon!