I always need to be working on something that I can all my own outside of my regular work hours. In the past these have been projects like Wordpress themes, small web apps and most recently dpadd - my social network and journal for gamers. I've been asked before how I get motivated and stay on task with my side projects. Here's a few tips I've found very useful over the last few years.
Pick something you're truly passionate about
I know this is probably at the top of every "get motivated to work on projects" article/post but it's just so important and has to be said. You have to care deeply about your creation or you're probably not going to get very far. You need to crave every spare minute you have to work on your baby. When I ride the ferry to and from work everyday I get frustrated that I can't write code or design UI on my iPhone. Make sure you pick something you want to work on more than playing video games or watching Breaking Bad. Working on a side project should be a hobby you love, not a chore that you need to talk yourself into doing.
You also need to have some passion for the activity you'll be performing to create your side project. I genuinely believe that anyone can code, design, write or what ever is necessary to complete their project - but if you hate doing it, you'll have a hard time even getting started.
When working set an end time (not just a start time)
I believe too many people are focused on "when they're going to start working on their project" and they forget to set a time to stop working on their project. Setting a mini deadline ("I'm going going to work for 1 hour") pressures you to focus and push yourself to work faster.
Don't wait for the stars to align. Just do something!
You can do a lot in 15 minutes. Seriously, you don't need 2 hours of uninterupted zen-like peace and quiet to work on your iPhone app or Wordpress theme. I read recently that people have a hard time getting started on web projects because they need a lot of time to get their head into the code, remember how everything works and plan out their next move. Bullshit. 15 minutes or less is more than enough time to fix a UI bug or sketch out a couple wireframes.
Keep a list of 'low cost' (cost being time) tasks in your to-do app, along with medium and high cost tasks. When you have a spare minute or two, pounce on something you know will only take a few minutes. These 'low cost' additions will quickly add up when you realize you can achieve a lot in the time it takes to brew a pot of coffee or fifteen minutes before going out with friends.
Also, don't wait to be inspired. Inspiration is great for ideas but not for getting down to business. The was a great quote posted in the 37signal's blog the other day: "I always thought that inspiration is for amateurs. The rest of us just show up and get to work." I couldn't agree more.
Find the right balance of challenge and achievability
Your project needs to be challenging but not out of your reach. This way you will constantely be completely tasks that feel rewarding, instead of tasks that are frustrating or boring. You should go a little, but not too far, out of your comfort zone.
At the end of the day, the genuine belief that you can accomplish what you set out to do is necessary. There will be tough times, stretches of dull work and of course, hating haters. But keeping a positive attitude through it all will be the difference between launching and not launching and will help it be an enjoyable experience the whole way through.