Like I said, there is a long list of tricks. Not all of them work for everybody but some are fairly universal. If you step back from the question a bit the issue is similar to other difficult things like losing weight or stopping drinking or whatever. People have come up with tools or tricks for these things over time.

1. Peer group, support group, etc. is very helpful. If you don't have anyone local to share your build with then post a progress report on here once a week. Do it for yourself, not for anyone else. You don't even need to read the replies if you don't want to, just make the post to show yourself what you accomplished that week.
2. If you are really stuck then set an easy goal and do it. Something as simple as washing the car can be a simple goal, or sweeping out the garage. Hitting an easy goal then gets you in the mood to tackle the harder goal.
3. Order just a few parts at a time so you don't get overwhelmed with things to do when the UPS shows up. I usually order $100 worth of stuff from Summit a couple of times of week. That way I always have some new stuff showing up. Sometimes if I'm stuck then getting a box of new stuff gets me going again.
4. Find some people in your local area who can get you unstuck. I have local resources who I can toss projects to if I can't get it done. Some stuff I just can't do like installing glass or covering seats. Other things like TIG welding I can do but I'm not good at it so I take it to a shop.
5. Crank up some music, sometimes that helps.
6. Invite a friend over in a week to show them what you finished up. That will give you a little bit of a deadline to hit.
7. Take pictures on a regular basis and then flip back thru them to see what you have accomplished. Post the pictures so you have a record that you can refer back to.
8. Ask for help. A little bit of help even from someone who is unskilled can be a big deal. I had a college kid come over and help me during the summer a few times. Even if they are just sweeping the floor while you are working on the car it helps.
9. Hire some help for the non-car stuff. Have a kid mow the lawn so you don't have to do it and stuff like that.
10. Turn off the TV
11. If you are really stuck then go get some exercise
12. If you are really stuck then go do something unpleasant that needs to be done like scrub the toilets, clean the oven, etc. Sometimes that will clear your head and get you motivated to go do some car work.
13. Go to a car show and wander around looking at other projects.

I could go on but I think you get the drift by now......