I do a lot of things to keep the hobby alive, but the one I feel is most important is teaching the games I play to other people. In regards to one of the rpg systems I play, I used to be the only person in this area (that I knew of) who played. As of yesterday, I am now involved in three seperate face-to-face games; two of them GMed by people who I taught the game to.
I also do my best to highlight the positive aspects of gaming. I do not get on a high horse or act uppity or anything like that when someone tries to bash gaming and start spouting all manner of rpg-lated urban myths (...it's the devil's game!) However, I do stand up for the hobby and present my case. I inform people that I have been greatly helped by the games I play. During my time in the military, I suffered severe head trauma which caused some of my mental skills to deteriorate; playing rpgs has helped excercise my mind and refresh many of those skills back to levels which are close to what they were prior to the incident. During a recent doctor's appointment, my doctor remarked that I've improved greatly when it comes to memory and various other things. Likewise, many of the younger kids I play with perform very highly in areas of math, reading comprehension, and problem solving.
Another of the things I do is do by best to support the local gaming store. While it is true that the local Barnes & Noble sometimes will have a product a few days before the local store, I usually wait the few extra days and buy the product from Gatehouse Games (the local gaming store.) Typically there's not that much of a wait, so it's no big deal. If I want something other than D&D, I'm usually better off going to Gatehouse anyway.
There are plenty of things I do, but those are a few of them.