Well, as Red said, I catch all post game legendaries I can, or go through any events left over that I can find. Then I try and find/train a Pokemon I want, such as Trapinch and evolving it into Flygon, or trading Rhydon and evolving it into Rhyperior.
Anyway, other than evolve, train, or hatch Pokemon, I just wait for wifi events to show up. However, when something like a new game is on the horizon, I'll just wait for it to come out before touching my DS, like around 4 months.