I struggle with procrastination/lack of motivation all the time with creative projects. This boils down to having little spare time and energy mostly due to the kids and work.
Opposite to Rob's post, I focus on one project at a time, not many. I keep a milestone in view, what do I want to achieve. Thats the goal. But really you have to find the way that works for you.
The best solution to defeating lack of motivation is "just get on with it". It can be the hardest thing to overcome, but once you've started you can keep going. A favorite writer's trick I know and I've used successfully is setting an alarm. Plan to write for 15 minutes and stop at the alarm. Once you've started writing, you may find you'll just ignore the alarm and keep going.
I find once I hit my imaginary milestone, I stop completely. But I find then is the point to start sharing it with other people and talking about it.
Keep a notebook or some sort of quick access data store and write down your ideas. I often use my blog to write up ideas (giving them slightly more structure).
and this is the key to breaking procrastination/writers block/motivation. My opinion is, forget all the best-case design approaches and get stuck in at the deep end, start with the bits you enjoy. As a coder there is an instinct to "do it right" but you end up doing all the boring stuff first and probably never finish it. Start with the stuff got you excited about the idea and work backwards.

Reduce distractions, if you can. Sometimes they are unavoidable (I have kids and my family always comes first). I'm programmer geek, I love the internet, I love TV and love books. If you can, cut it all away and it's much easier to get started.
But I also offer this counter point: for me, I kinda need my crappy TV to detune from work and other commitments. Sometimes it inspires me. A book I read on inspiring writer offered this advice, keep what inspires and encourages you to write and try to ignore which doesn't. Try to figure out what type of books, movies, friends, etc. motivate you and try to cultivate them more. For me, sometimes watching a bad movie or reading a weak book inspires me to write because I know I could do something much better. An old friend who has the completely different style of gaming and who constantly argues with me about the nature of roleplaying, turns out to inspire me to design better roleplaying games. You have to find what works for you.
And finally, do it for yourself. Enjoy the process, enjoy the learning. If you can succeed at that, all other advice is moot. You'll never need a real audience. Two of my favourite authors never found real success in their lifetimes but they were prolific (HP Lovecraft and Edgar Allen Poe). My Lost Heroes RPG project took so long, because I ended up enjoying doing the research and cataloging and working through the different mythologies. I did my own god family trees from the stories I read rather than re-use ones already created. It became a passion. It changed the way I looked at world in general.
Another reason to do it for yourself is, there are always room to improve. Sometimes I come across a book or a rpg and I'm amazed at how great it is. This ends up becomes quite demotivating. You'll never be Shakesphere, your project will never be that good (it could, but you feel that it won't). But you got remember sometimes it's luck that brought all those factors together. Strive to always better and and improve, don't do it to simply show off to others how great you are. If you can get good critical advice (not necessarily positive advice), heed it and ignore the bad advice. Rewrite. Appreciate the skill of design and writing.
I think I wrote more than I meant there (maybe I should rewrite this post a bit better... nah!)