"Who's Whom?" Just kidding, I'm voting for "who," but you're free to vote for whomever you like. Seriously, though, here's how to remember it. If the word in question comes in the form of a question, just supply a plausible answer. If it's "I can trust him (or her)," it should be whom. There were 24 cases of Covention* in OhioLINK, and 544 in WorldCat.
(Photo of Adam West as Batman and Burt Ward as Robin, 1971, from Wikimedia Commons.)