Oh, that reminds me, two other things I'll do.

1) Check posting history. If they have a habit of jumping out of RP's, keep that in mind.

2) Reject some people for [i]absolutely no reason[/i] (I can make up bullshit as well as the next guy) and wait 3-4 days to get back to that person. This tends to vaporize 50% of the problem players right there, since most of the impatient ones also tend to have fragile egos. Bonus points in that it also works against people who cannot stand criticism, which if you have a long lasting RP, will invariable occur to everyone at least once.