On search, I've gotten upto as high as 40% consistant. But the keywords I was targeting were exact match and there isn't a lot of competition.
But honestly, it's mostly common sense. I'm 2 fucked up right now to go into detail, but a good start is to organize your ad groups and create hella specific ads. If there's enough volume to justify it, create an ad for every keyword in either phrase or exact match. Just always be hungry for more CTR.