I'm afraid I cannot recall who said it - I tried looking it up and googling different keywords, but nothing came up - that if the reason you're doing something is irrational, you're more likely to stick to it. Say you have a principle that you don't smoke. You could easily try to reason your lack of smoking with a variety of reasons - it gives you lung cancer, it decreases your lifespan, you start to smell bad, your voice gets all coarse, you'll set a bad example for your kids, you get irritable if you can't get a cigarette - and this is probably what most people who don't smoke do.
But what if your reason was none of those? What if your reason was, because there's a gigantic omniscient muffin-eating teapot flying around in the sky that will know automatically if I have a cigarette and come down and slap me in the forehead with that muffin if I do?
And what if your reason wasn't even that, but because I just don't smoke?
It's funny to write about this since I used to think everything Ayn Rand says is pure gold, and she has a quote that says, "to irrational principles, one cannot be loyal." (The quote continues with "ideas that are not derived from reality cannot be consistently practiced in reality", a statement which actually isn't a straight continuum from the first.)
Anyway, let's start with the muffin-eating teapot. (Yes, it's a Bertrand Russell reference.) Anyone would think you're insane to believe in such a teapot, and you probably know rationally that there's no vengeful teapot anywhere, and that is why you yourself think it's an irrational motive to stick to your non-smoking principle. With any of the "real" reasons you can start to rationalize and think like well, my kids can't see me right now, or it's just one cigarette, I won't die, or I'll wear a plastic bag and plastic gloves and brush my teeth afterwards to the smell won't stick, and so on. And soon enough, you've abandoned your principle altogether.
But can you rationalize the teapot?
You can always say, well, the teapot isn't there! I'm off the hook! - which renders the whole irrational reason paradigm pointless. But if you say, it's just not something I do, there's no room for rationalizing. You have no argument to work around. In a way, this one, too, is an irrational principle. As a bonus, your cognitive dissonance will mirror itself from the principle (in your mind) to your actions (in your physique).
It doesn't get more convenient than this.