My approach is, frankly, to use friction to keep my goals. I have a set of activities I do and pursuits that I pursue and eventually I grow tired of each. I rock climb three times a week for a few months, get really strong at it, then burn out a little and back off. I sign up for big cycling events (another good one -- make external commitments) and then train for them, lean out and get my heart in great shape, and then afterwards, slow down on it. There's only so many weekends in a row I can bike 50+ miles before I'm just like, meh, and slow down.
I haven't been on my road bike in months, but earlier this year I was training my ass off.
Then I slack and eventually am like, ugh, I gotta get my ass back on the rock, or my bike saddle, or on my meditation cushion, or in my yoga classes, or in an apron in my kitchen or whatever, and so it goes.
I'm never just sitting around doing nothing, I just do too much to really do it all well at once.
Eventually when things are important to me, I can tell, and I pay attention and the motivation comes.
Here's another little anecdote I liked, though, that touches on motivation at a longer-term level. A Buddhist teacher was giving a lecture, a friend of mine had a recording, and he's this guy who's been very successful at his pursuits, opened a retreat center, wrote a book, etc. He sets his mind to things and gets them done. Someone asked, during Q&A, how he was so prolific and successful at his undertakings and he said,
"Well, I realized at some point that any real project I undertook was about a 5-year commitment. I'd start thinking about the book, and later really start putting together notes, and even though the actual writing only took a handful of months, from start to finish it was five years. The retreat center, at first, I thought it would only be about a year or two, but finding the right land, then getting good teachers to draw students, building word of mouth, everything it took was about five years start to finish.
Once I realized that, I realized I know how many projects I have left in me. If each one is 5 years, given how long I'll probably live that's around 6 more projects, maybe as many as 8, but who knows how physically capable I'll be for the 7th and 8th.
Knowing that I only have 6 more real, solid projects in me, it makes it very easy to be work diligently and stay focused on each one."
I thought that was kind of sobering, but at the same time, I see his point.