What I find it best to do is to be really excited about it when it's first announced and when the first batch of info comes out. Then, a lot of people forget about it and the game loses some of its "juice" as you said. But regardless of how people forget, they just need to be reminded by a release date. Usually I find it best to remind people a month before it comes out and then everyone I know gets really hyped up about the game again for that last month of waiting. Then, since they can't wait, they spread the word, too. It's a great chain reaction!
Danny