There is a lot of talk here about sophisticated rationality failures - priming, overconfidence, etc. etc. There is much less talk about what I think is the more common reason for people failing to act rationally in the real world - something that I think most people outside this community would agree is the most common rationality failure mode - acting emotionally (pjeby has just begun to discuss this, but I don't think it's the main thrust of his post...).
While there can be sound evolutionary reasons for having emotions (the thirst for revenge as a Doomsday Machine being the easiest to understand), and while we certainly don't want to succumb to the fallacy that rationalists are emotionless Spock-clones. I think overcoming (or at least being able to control) emotions would, for most people, be a more important first step to acting rationally than overcoming biases.
If I could avoid saying things I'll regret later when angry, avoid putting down colleagues through jealousy, avoid procrastinating because of laziness and avoid refusing to make correct decisions because of fear, I think this would do a lot more to make me into a winner than if I could figure out how to correctly calibrate my beliefs about trivia questions, or even get rid of my unwanted Implicit Associations.
So the question - do we have good techniques for preventing our emotions from making bad decisions for us? Something as simple as "count to ten before you say anything when angry" is useful if it works. Something as sophisticated as "become a Zen Master" is probably unattainable, but might at least point us in the right direction - and then there's everything in between.
I suspect there's a bit of both going on but I'm fairly sure it's not as dramatic a discounting as an 80% to 1% change (I realize your numbers were only illustrative of the idea). My feeling based on introspection of the decision making process when making a choice that favours short term gain over the more 'rational' longer term choice is that I am still fully aware of the negative consequences, I just discount them heavily.
If there's one area where my judgements are distorted it is in my estimate of how likely I am to be able to 'make up' for present choices in the future. I think this is a fairly universal phenomenon and is also reflective of conflicts between present and future selves - I may eat the chocolate bar and commit my future self to exercise or a healthier eating regime but I am far too trusting of my future self and consistently underestimate his incentive to renege on any commitments I attempt to bind him to in the present.
In my personal history I have an unusually explicit example of present/future self conflict. When I was at university I made short term decisions which I explicitly justified to myself and others on the basis that I was making choices that my future self would have to pay for but that I anticipated my future self being a person who my present self would have no qualms about taking advantage of. I was aware of the fact that political views tend to move further to the right with age, as best expressed by the line "Show me a young conservative and I'll show you someone with no heart. Show me an old liberal and I'll show you someone with no brains." and as a young liberal anticipated an older and wealthier conservative self who might not believe in wealth transfer. By taking out student loans I could commit my future self to a wealth transfer that suited my purposes at the time but that my future self would likely not approve of.
As it turns out, I was at least partially right about where my political views would move (though of course if I met my younger self now I would attempt to point out the many rational reasons why my views now are in fact more correct, and the many ways in which his understanding was overly simplistic). Overall I don't begrudge my younger self the choices he made however, though that may only be because the commitments did not prove to be overly burdensome.