Matthew Might has an excellent article on his website.
But what are some other good suggestions? One important thing is identification (some professors are simply happier to reply to emails than others). Surprisingly enough, I find that famous scientists in my field (for whatever reason) tend to be more responsive to emails than non-famous scientists. I don't know why this is the case, but maybe it could be that they simply know more, so it doesn't take them as much effort to give information/advice?
Sometimes, I even go as far as to look at Rate my Professors site ratings, since the ones with exceptional ratings often tend to be happy replying to emails.
Another thing: maybe taking a class with the professor (in which case they may feel a bit more obligated to reply to them?) Or maybe simply distributing one's emails in such a way that one wouldn't have to email each professor more than an interval of once every few weeks?