My wife and I work on software projects. They become fairly advanced because she codes and I PM / test. Every time she implements a new feature or fixes a bug, I'm happy because the bus is moving forward, and she's happy because she's made me happy. I also update her on happy user stories, and make it obvious they make me happy because once again, it makes her happy. Anyway, that's just one way. I'm sure there's others.

(ETA I'm a woman, I realized this could read badly otherwise :)

Another time, I wrote a novel, and she read each chapter when I was done. I realized that if I didn't finish the novel, I would leave her hanging, and I didn't want to do that, so I finished it, even when I got 'mired in the middle'.

I guess my point is, try to involve someone you care about to hold you accountable, make a commitment to them to finish it.