Motivation often comes from witnessing the positive results of your actions. A rationalist is especially attuned to this form of observation so it would seem that exercise is the perfect arena for the rationalist to succeed. I run and lift weights and I feel and perform better (disregard looks for now because it's too loaded). If I stop, then I feel and perform worse. Therefore, as a matter of rationalist discipline I will continue to exercise. Eventually, this should normalize into a sense of motivation. For the first timer, exercise might feel like hell but over time a positive and motivating association should develop.
I would illustrate that like this: exercise (don't like) = feel and perform better (like) => exercise (like) = feel and perform better (like)
And I don't think fat people are irrational, just undisciplined. Developing habits and mental associations takes time. Doing things you don't like over a period of time (which is the same as giving up something you like in the short term) in the interest of a deferred goal is the definition of discipline.
Robin wrote how being rational can harm you. Let's look at the other side: what significant benefits does rationality give?
The community here seems to agree that rationality is beneficial. Well, obviously people need common sense to survive, but does an additional dose of LessWrong-style rationality help us appreciably in our personal and communal endeavors?
Does LessWrong make us WIN?
(If we don't WIN, our evangelism rings a little hollow. Science didn't spread due to evangelism, science spread because it works. Art spreads because people love it. I want to hold my Art to this standard. Push-selling a solution while it's still inferior might be the locally optimal decision but it corrupts long-term, as many of us have seen in the IT industry. That's if the example of all religions and political movements isn't enough for you. Beware the Evangelism Death Spiral!)
We may claim internal benefits such as improved clarity of thought from each new blog insight. But religious people claim similar internal benefits that actually spill out into the measurable world, such as happiness and charitability. This fact gives us envy and we attempt to use our internal changes to group together for world-benefitting tasks. To my mind this looks like putting the cart before the horse: why compete with religion on its terms, don't we have utility functions of our own to satisfy?
No, feelings won't do. If feelings turn you on, do drugs or get religious. Rationalism needs to verifiably bring external benefit. Don't help me become pure from racism or somesuch. Help me WIN, and the world will beat a path to our door.
Okay, interpersonal relationships are out. Then the most obvious area where rationalism could help is business. And the most obvious community-beneficial application (riffing on some recent posts here) would be scientists banding together and making a profitable part-time business to fund their own research. I can see how many techniques taught here could help, e.g. PD cooperation techniques. If a "rationalism case study" of this sort ever gets launched, I for one will gladly offer my effort. Of course this is just one suggestion; everything's possible.
One thing's definite for me: rationalism needs to be grounded in real-world victories for each one of us. Otherwise what's the point?