The key to improving Aikido is daily practice. I say it all day. But that is not enough. One spends so much time and effort acquiring any skill that once a certain level is reached it is easy to become complacent or to not take new information on. This is the beginning of stagnation. Everyday one must come in with something in mind to sharpen or to focus on a weak area. This way we keep fresh and at the same time condition ourselves to practice excellence.
True Aikido is a level of virtuosity above what would normally be considered good. As a martial art Aikido can be functional at a lower level but will not become transcendent until excellence has been attained and maintained. Real Aikido soars above the techniques and the movements of the art. It becomes High Art, philosophy mastery. One has to go through the normal levels one would in any martial art but also go further.
The key to improving Aikido is daily practice.