Just thought of a new one, and I particularly like it:
Bragging about your IQ is the intellectual equivalent to bragging about your penis size.
I could expound on that in a whole variety of ways (such as “Bragging about your IQ in an intellectual debate is the equivalent to bragging about your penis size in a fistfight”, or so on), but the point is clear: it’s stupid, so don’t do it. No-one who knows how IQ tests and results work put much credibility in such things, anyway; sure, they measure raw brainpower, but such is rendered virtually useless if you don’t have a good grip on reason, logic, and a critical mind to support it. For an all-too-apt example, just look at my favorite punching-bag, Vox Day. The wanker’s intelligent, no doubt, but he also can’t pose a decent argument without making half-a-dozen fallacies in the process.