Maintaining or increasing your English level?

You  never stop learning a language because languages are like vast ever-changing living ntities. 

Once you have achieved a certain English level the worst thing you can do is to attempt to maintain that level.

You must never be happy with your English. When you do you stagnate, and when that happens eventually your English "fossilizes" which is the same as going back to basics. That means no English at all.

To avoid that you must remember that learning a language is not like learning how to ride a bicycle. If you have achieved an acceptable  level of English you should aim to increase it not maintain it. For that all you need is commitment.

Learning English is a life decision, and that is precisely what you have to do: make English part of your life. Thus, you will have the necessity to speak the language on a regular basis, and that will make the difference.