I have been playing piano for about 5 years before quitting last year, due to financial reasons. I still can play some pieces, but how could I potentially continue piano with improvement without a teacher? The last few pieces I played were: Pathetique Sonata (Entirety) Rachmaninoff Prelude G minor Sibelius Romance op 24 no 9 Clair de Lune, Debussy
There are a lot of ways to continue improving your piano skills. For one, Music Theory.net is a staple in the music community for lessons, exercises (very important) and tools to aid you in maintaining your piano playing skills.
Secondly, play pieces which challenge you musically and, perhaps, physically. I can't possibly list them all but there are a bunch of places online that have free sheet music for pretty much any song you can think of. And they are printable. Assuming you have a printer this is great. If not, you can always print them out at a local library.
Third, play everyday. A famous quote by Jascha Heifetz, "If I don't practice one day, I know it; two days, the critics know it; three days, the public knows it."
Don't just noodle around the keyboard when you sit down. Make your piano sessions substantial. Do exercises. Try a new song. Write your own songs. As a guitar and piano player of 11 years I can tell first hand that these things will improve your playing as long as you stick with them. Hope this helps -Daniel