I have completed three apps under similar circumstance. It's tough. It's really tough. Here's some stuff that helps me:
- The hardest part for me is getting started at night when I'm already tired and brain-drained from a full workday. Once I'm working it's ok. It takes habit. Such as, every day at 9pm or whenever you start just sit down with xcode and built clean five times without changing anything. Then you can go play games if you want (but I find I usually don't if I can get that far).
- You can't do it with just small blocks of time alone. You can do most of it, but you HAVE to have a few eight or ten or sixteen hour marathon days in there to tackle the big parts. This is especially good when you're running low on motivation. It doesn't take a lot of these marathon days but at least for me I can't finish anything without a little bit of self-imposed crunch.
- Don't start small. That's aiming low and for me that kills my motivation. Start with a impossibly big idea and then when you start to get overwhelmed cut features until you end up with something small.
- Don't tell people what you're planning. Only show them what you've done. For me, what worked was not allowing myself to tell them about the app (not even mention it) but I could demo it to people to show off what's completed.
- Work in the mornings if you can. I can't do this at the moment but I was getting up at 6 for a while and working on projects until 9 and that really helped.
- Only work on one thing at a time. If you start getting sick of it, too bad. Cut features until you can ship it. This is brutally hard to do because you're trading the potential for what it could be and what you see in your head for some crappy half-baked version. But what's hard to realize is that other people don't have that same vision of it as you do. The "half-baked" thing might be pretty cool to them. When I'm at this point I make index cards on a corkboard with the features I want and then cross them out (if I implement the feature) or remove them (if I cut it). I don't allow myself to add any cards until I've shipped an update. Ship it.
- Never rewrite your bad code (well, if it at all works). Do it right next time, live with your mistakes.
- Find others. It's really helpful to have others to talk to and share progress.
Also don't get your hopes up about making lots of money. It can happen but most apps don't make much. I'm making just enough to pay for my apple gadgets, and since it's kind of a hobby that's OK. But you're right in that it's a good skill to have and in high demand right now.
I'm procrastinating working on something like this right now. Sigh.
Good luck.