Look around for posts by Bernhard, he's posted many great ideas on just this thing. In a nutshell - it boils down to this:
1. Practice every piece you want to maintain in your repertoir, every day, forever.
-or- if that sounds insane

1. Use all the tools you can to learn a piece the first time through, and keep careful track of the process you used to learn it.
2. Forget the piece for a time (6 months, a year, whatever)
3. Return to it, and go back to your notes from way back and re-learn the piece following the same steps. Of course this time it will take you much less time, but be sure to follow the same steps.
4. Forget it once again for an extended time.
5. Repeat #3 (taking even less time to do so) and you'll have it for the rest of your life.
I think you'll also find that Bernhard agrees (as do I, and apparently the other responders) that you also should pick pieces you enjoy. Why waste time learning pieces you can't stand playing?
Lastly, you can impress people with beautifully played yet simple repertoir. Don't get ahead of yourself just "to impress" as that will likely yield just the opposite.
-Paul