Fortunately I had to deal only with few of them (until now) but, it's always been an painful experience. Especially for a natural born No Man like me. You can easily detect Yes Men by looking at the strength they use to defend their boss' ideas, no matter how stupid.
The funny thing about Yes Men is that they think to be a step higher than everybody else just because they always agree with the boss. For this reason they are totally advice-proof. No matter the arguments you put in your speech, they simply ignore everything is different from what they think is their boss idea.
And with "ignore" I mean that they don't even try to tell you are wrong. No explanation is provided. It's just like you are a kid that said stupid things just for fun: there's no need to talk, you'll grow up.
Unfortunately for them, when the boss agrees with you, they lose the ground beneath their feet. But they rearrange their mind quickly because
Only stupid people don't change their mind.