I only have so much energy after waking to get started with anything creative like project design, so it comes down to managing that. One trick that works is waking up at 5 AM to work on a personal project, then going to work at 9 AM. This might lead to being pretty slow at work once afternoon rolls in though. Another way is to do an early workday, 8 to 4 or 7 to 3 even, then take a 20 minute power nap after work. Even a momentary falling asleep can restore a surprising amount of mental energy.
For me it has not that hard if you have a lot of passion for what you are building. dworrad's post the other day is a great example of how he does it. It feels like a bit of a obsession - check it out.
I am starting to think I have some sort of OCD with this project. Maintaining motivation is the hardest when you get discouraged.
For me I had to give up all my other hobbies. I take care of my family (including 2 kids) do the best I can for my job but try not to work much more than 40 hours, and then its my side project after the kids go to sleep, from 9-1am at least 4 days a week. Wake up early or stay up late. If you are obsessed like I am you will wake up with no help at 4am, which is a great time to code.
I like to keep a todo list with time estimates next to each item. In the morning, pick one you'll have plenty of time to finish. Work on it, commit it and go off to work. It's a long game, but it works.
This depends on what your full time job is and the scale and intensity of your side job. Make sure that your plate isn't filled up 110% and know that people tend to underestimate the amount of work involved in doing anything...
I yesterday documented my struggle in this department while trying to get my live sports commentary site www.commentary.io up and running.
It isn't easy when you are a "sole founder"... especially when you have a family!