I used to have this a ton; not only with RPG's, but game projects in general. What solved it for me was to never actually start working with the project on my computer before I had spent a lot of time sketching and writing down ideas for the game, on paper. And then, when I felt ready to begin working on it, I let it sit there for a week or two, and come back to it. If I didn't feel like continuing the project at this point, I'd just let it sit until I eventually did, or I used its ideas for a new project I felt more like working with.
So now I have two games; one is a platformer made with my own engine in Flash, and the other is the RM RPG game I'm working on. The platform game is on my backburner, but it's a "simpler" (at least story and gameplay wise) game than my main RPG project. When I come up with new game ideas I sketch some and write some, and let it sit there, but more often than not elements of them fit perfectly into my current projects.