The best clues to me are:
2. Very slightly cryptic in the way that they make little sense until you’re at GZ. As in “42” for something bearing a number or “I wear a china hat” (For a flowerpot).
3. Don’t spoil the discovery. The best hides are those that are pleasantly surprising when found, aren’t noticed by muggles but can still be found by almost everyone who /is/ looking.
I don’t like clues that:
1. Don’t exist. I know there are many who believe that field should be entry, and cite “back to basics”, but it’s there for a reason. People can read it if they want or not, but give them the choice!
2. Rely on photos or websites. As an extra, fine, but not instead of.
3. Need trivia knowledge not everyone will have, unless that’s a part of a field puzzle. I love latin tree names but understand that those who don’t know them you might as well be talking japanese.
As a cache owner, I know that the only way not to get people leaving irritated DNFs is to spell out exactly where it is. If you try to be funny, clever or inventive, somebody will get uppity with you… One day I might even get it right!