I'm sure there are specifics, but in general, be in good standing with the majority of the clan you're trying to join (i.e. don't be on any of the cores' bad side). That's probably the most important.
Be active and probably be capable of doing mid-endgame to late-endgame content if you don't have connections within the clan to vouch for you.
Learning the politics of the server is also helpful. Know who's allied with who, and which clans are warred with each other, as that may play a large part in your decision.