How do I maintain my improvement.

In the past year  my rating has increased from 800 to 1500. How do I maintain this?

Keep playing studying and watching chess

Do Tactics!! Study positions!!Study Endgames!!!Play more games,to gain more experience!! 

Simply play unrated - win, lose or draw, your new, much cherished, rating will be maintained.

IMO the basic areas are tactics, endgames, strategy, openings, and annotated game collection.

Pick your weakest area and devote serious study to it. For example purchase a well regarded classic book on the topic. (by annotated game collection, for example Fischer's 60 memorable games or Zurich 1953).

Covering all your basics in this way will get you around 2000 chess.com rating.

After that I'd say it's more of the same, except extra emphasis on looking at GM games, both historical and modern, and not only top games but lesser GM tournaments too. You have to analyze a lot of quality positions, and play a lot of games.

Oh, I might have misread the question.

I thought he meant how do I keep improving at a fast rate.

Yes, most people want to improve, so normally your advice would be correct, Llama. But not this guy though!

Haha

study your booty off - puzzles before game play and don't play when you first wake up, when your annoyed, hungry, or distracted in any way - one day at a time