First, I concur with /u/maia's suggestion: Find a partner. (Categorize that under "Shame," because you will be ashamed to quit when someone else is looking. Or file it under "Fun," because it can be a lot of fun to exercise with others.)
Second, you may also want to pay attention to the unspoken, automatic thoughts you have when you are losing motivation. Try to articulate them in sentence form so they can be interrogated. (For example, one might be something like "I'm never going to make any real progress" or "I'll always be a quitter.") Having identified a few, think up a good response sentence (e.g., "I won't make immediate progress, but I never expected to. If I keep working at it every day, including today, I will almost certainly be pleased with the results" or "Just because I've quit a lot in the past is no proof that I must always quit" or "Only the final attempt at habit-forming is ever successful, so it's natural that the losses outnumber the wins. No need to get down because of them, but I still want this attempt to be the winner, so I'll work at it again today.") Write down the response sentences on index cards, memorize them, and repeat them to yourself often (and especially when you sense your motivation slipping.) See if that's helpful, and do report back.
(These come from my understanding of cognitive behavioral therapy.)
you may also want to pay attention to the unspoken, automatic thoughts you have when you are losing motivation. Try to articulate them in sentence form so they can be interrogated.
As best as I can recall, they're roughly along the lines of "This is gonna hurt" or "Ow" or "Yep, that was painful". (Mostly during push-ups and planks.)
(Though at one point, I wondered if I could make a Super-Happy's head explode by presenting them with the idea that I was deliberately and voluntarily choosing to engage in an activity that cause...
"Cryonics has a 95% chance of failure, by my estimation; it would be downright /embarrassing/ to die on the day before real immortality is discovered. Thus, I want to improve my general health and longevity."
That thought has gotten me through three weeks of gradually increasing exercise and diet improvement (I'm eating an apple right now) - but my enthusiasm is starting to flag. So I'm looking for new thoughts that will help me keep going, and keep improving. A few possibilities that I've thought of:
Pride: "If I'm so smart, then I should be able to do /better/ than those other people who don't even know about Bayesian updates, let alone the existence of akrasia..."
Sloth: "If I stop now, it's going to be /so much/ harder and more painful to start up again, instead of just keeping on keeping on..."
Desire: "I already like hiking and camping - if I keep this up, I'll be able to carry enough weight to finally take that long trip I've occasionally considered..."
Curiosity: "I'm as geeky a nerd as you can find. I wonder how far I can hack my own body?"
Pride again: "I already keep a hiker's first-aid kit in my pocket, and make other preparations for events that happen rarely. How stupid do I have to be not to put at least that much effort into making my everyday life easier?"
Does anyone have any experience in such self-motivation? Does this set of mental tricks seem like a sufficiently viable approach? Are there any other approaches that seem worth a shot?