Collective Momentum
Usually, all people involved in an idea initially show equal excitement. It's new, it's interesting and none of the roadblocks have shown up yet. As soon as hard work is required, typically past the initial brainstorm sessions, it takes greater effort to stay focused on a project, especially when other work will pay you. If even one person in a group starts to stray, it creates a ripple effect that leads to others questioning the idea as a whole and fearing failure before anything is even delivered. The best way to combat this is to meet regularly and have very specific milestones you wish to reach. It also helps to only bring people into the group who are known for dedication and reliability.
Build Something Fast
Regardless of what you think of agile and lean idealogies, having something that people can see allows you to recruit people faster and get people interested. No one cares about your ideas. Everyone has ideas. Geniuses execute. Sitting around and telling people about your next startup will keep them interested for about 5 minutes; showing them a rough of your new app on your iPhone will get them asking questions and remember your idea. As a developer, I'm guilty more than others of hesitating to prototype because I'll most likely have to rebuild what I've already done. However, if you get in the habit, you'll get feedback faster and increase your chances of building less garbage later.
Write Copy Into the Live Site
At RentHackr, one of our meetings required us to hammer out a substantial amount of copy that was at the time in the form of 'lorem ipsum'. Initially, we started writing on a whiteboard and Google Docs, then we just decided to hook up one of our laptops to the big screen and edit the HTML on the fly. This allowed us to see the copy blocks where they would actually exist. Immediately, we were able to see elements that were too long, headers that didn't stand out enough, etc. Aside from improving the copy, we were able to feel a greater sense of completion by getting it into the site faster.
Stop Brainstorming
Brainstorming is great. I love it. I encourage everyone to do it even in a traditional work environment. However, at some point, you have your idea and you need to build something. One of the problems I ran into with several very talented friends was stumbling upon a great concept, then slowly discussing the life out of it. This leads to the core of your idea disappearing while you endlessly discuss less important details. At some point, you lose sight of your original idea and (just like #1) momentum is lost.