The only sure fire trick that works for me is to have a helper. If a buddy stops by to help me with the car we get a lot done. If nobody is available to help then I hire someone to help me with the car. If I'm paying someone to work on the car then I get motivated too. If I'm all by myself then the work proceeds in a somewhat random fashion where I pick and choose what to work on. I'll get focused if I have a deadline for a magazine article but self imposed deadlines don't do much for me.

I have learned to stay busy doing something productive even if I'm not working on the car. I might clean the garage or tidy up the tool box or do something along those lines if I'm not feeling the energy to work on the car. Sometimes that helps. The other trick I'll use is to buy a few parts that I need. When the parts show up I get motivated to put them on the car.