It turns out that over the years I've done a lot of indie and small press projects. I've had people ask me how I manage to get these things finished; what follows is my best advice for cleaning your plate when you've got a creative project in mind. I make no promises that it will work for you, but this is what works for me:
You Have to Want to Do the Work
You have a great idea for a game, setting, adventure, or supplement. It is a shining, perfect idea. But it will remain an idea unless you actually want to do the work to make it an actual thing that exists in the world.
I know a guy who has been talking about the great movie he's going to make for over a decade, and he hasn't filmed a second of it. He's never going to make that movie. He likes talking about his plot, the themes, and how great the casting will be. But he doesn't want to do the work, so that movie is never getting made.
Don't be that guy.
Before you start talking about your great idea, before you start making plans about where you're going to publish it and how you're going to market it, do a little reflection and make sure you actually want to do the work necessary to keep at it until it is done.
Survey the Field
Okay, you want to do the work, congratulations! But before you roll up your sleeves and grind it out, I think you should really dig into what other people have done that might be similar to your project. Surveying the field will help you find models of how to approach your project, and also give you ideas about how you can make your project stand out from the pack.
When you're surveying the field, do yourself a favor and look at products outside your comfort zone. If you're an old-school D&D fan, look at what's happening in the "storygames" community too, and vice versa. Look at everything. See what is out there and learn from it. If you have an informed idea of what format will work best, what design decisions will need to made, and what you want to include or exclude going into your project, you'll have already cleared one of your project's major hurdles.
Bite Off What You Can Chew
Break your project into small, bite-sized chunks that you easily accomplish. No, even smaller than you're thinking right now. Give yourself assignments like "I'm going to write up three NPCs a day" or "I will finish one random table a day." One "one NPC" a day is fine, really.
Make each task small enough that you can complete it and will have time and energy left over to do more than what you scheduled for. The feeling of going past your allotted goal will make you feel like a champion and feeling like a champion will keep you motivated to keep working because you will see yourself making progress.
Small, incremental goals are also easier to "make up" if you miss a day's goal.
Make Your Lonely Fun a Little Less Lonely
Have your smart and talented friends give you feedback on your work while it is in process. Resist the urge to only show them a "polished draft"; let them see the warty, horrible thing that you are semi-embarrassed by. The feedback and suggestions they give you in the earlier stages is crucial; they will tell you useful things that help you right the ship before it's already on a collision course.