I show everyone that shows the slightest interest what I'm doing. I make incremental progress then, as a reward, work hard towards specific milestones on my nights off; my wife and I trade nights where we don't have to be a parent and get to go off and play and I often just go to a coffee shop and work on my side project.
My slow progress can be extremely frustrating but I try to offset that with the gratification when I achieve my milestones. I used to doubt that I could be this disciplined until this particular side project; now it's effortless because I'm doing what I want to do and it makes me happy.
And, ultimately, that's what decided it for me: am I doing what makes me happy? That's motivating for me. Not money, not fame. That's why that 6-9 family time is pretty inviolate and I tend to spend all my time with them on the weekends. Ultimately, my family makes me happier than my side project so they get the majority of my time and thought.
But that side project is always waiting to be worked on. ( =
Then work late and sleep in :-) I can't do a lick of useful work before 10 (which isn't great, since my job starts at 8:30), but I can cram 2-3 hours of good stuff in after my kids go to bed every night. Try different times to see when you're at your best. It's worth it to feel like you're making progress on something awesome.
And I've got to echo seiji - just get your product out there as quickly as you can. Let your coworkers look at it. Send it around. It may be that no-one wants to use it in its present form, but just looking at what you're doing may shake loose ideas and directions for the project that never occurred to you. Getting outsiders looking at your work is the difference between building something neat and building something great.