I have had decent luck pairing a hand signal with the word "wait." For some reason, the hand signal seems to convey what I want more so than the word at first. I hold my hand up in front of them, palm facing them, really just like you would tell a person to wait. If they do it just for a few seconds at first, then I say "good boy!" or something like it so that they know they got it right, and then reward them with something yummy.