Whether it's because you blush bright red or because you contradict yourself immediately, the result is the same: anyone you might hope to lie to will very likely realize that you are lying to them. An ineffective lie is a lie you have no reason to make: you gain nothing by it.
And if people know that any attempt you make to lie will fail, they will know that you are telling them the truth. People will trust what you say; you will find it easier to convince them of things. In the general course of everyday life, I think, you will be better off than if you had the option to lie.
This claim is surprising, because in any given situation, you are clearly no worse off if you have more choices -- if, in a particular case, you were given the option to lie, you could always still choose to tell the truth, so merely having the option to lie seemingly can't make you any worse off than you'd be if your only choice was honesty. The resolution to this paradox is that the knowledge that you will not lie in the future affects the choices others make, and therefore can be of use to you know. Game theorists call this sort of useful precommitment a "threat."
Merely trying to be honest each time you have the chance to lie is a risky strategy, in the sense that when the reward becomes high enough, you may well resort to deceit. But if you train yourself to be an incompetent liar, your integrity becomes priceless -- no matter how much you might want to sell out, you can't. And if this fact is known, then you'll be just as trustworthy -- if not more -- as you would be if you really had principles.
Which is to say, you can threaten to be good.
Taking such a claim at face value makes sense: if you really are a poor liar, then, as seen above, whatever you claim will be true, including your claim to be a poor liar.
But then again, you might instead be a terrific liar. And in this case, your claim to be a poor liar is itself a lie -- one your superior lying skills will enable you to get away with.
At first, I thought these two cases exhausted the possibilities. But then I realized that there's one more: you could be an okay liar, one who can get away with some lies, but not all. If this is the case, your claim to be a poor liar is a lie -- except that unlike the terrific liars, you can't get away with it. As you explain how you always blush when you lie, you start to blush. This tells me that you're a good liar -- you can get away with some lies, no matter what you say -- just not good enough to lie about this particular issue.
To recap, if you claim to be a poor liar, it could be that you can't lie at all, or that you can lie whenever you want, or neither. That about covers it.
Consider the classic thought-experiment: would you lie to the Gestapo if they knocked on your door looking for the Jews hidden in your attic? In this case, by telling a lie, you save lives; by being honest, you collaborate with evil. Which is more important -- abstract honesty or human life? Unless you're a hard-line Kantian, you probably value life over your social obligations to avowed murderers. So you'd lie, right?
NO!!! WRONG ANSWER! NO! NO! NO!
Look. If the Gestapo (or their moral equivalent) find out that you hold such principles, they're not going to believe you when you tell them there's no one hiding in your house. The whole point of telling them there's no one inside is for them to trust you enough to go away without checking. If they think you're the kind of person who'd lie in a situation like this, your answer won't have any weight with them. They've got to think that you're a hard-line Kantian who'd never ever lie, not even to them, or your lie won't work.
It's fine to believe that other, more pragmatic, concerns can justify lying -- but if you're going to be pragmatic when the knock comes, you ought to be pragmatic now, too. And that means not letting yourself get the reputation of someone who'd justify lying on pragmatic concerns. Your desire to help the Jews hiding in your attic demands nothing less.
To review, from 3, in order to make make ethically justifiable lies work, we need not to be known as people who would lie in those circumstances. From 1, we are better off being known as incompetent liars than merely as principled non-liars. From 2, we cannot acquire this reputation through anything we say. Therefore, we need to go all the way and make ourselves into people who are physically incapable of lying convincingly.
Except that this means that when we say "No, there are no dissidents hiding in my basement," the secret police will know we are lying and search the basement.
Back to the drawing board.