What I do is send them clothes before 5 pm (the time before the second mail comes for the day) then save/quit. Once it's over 5 pm, I load the game then check to see if they're wearing it. If not, I exit to home menu then launch the game again. It won't save as long as you won't so I repeat the process until they wear it. Once satisfied, I save it. That is 100% effective, at least for me, so no need to send two or more of the same clothes hoping they wear one of them. This is obviously a bit harder (well, tedious) if you're trying to make more than one villager wear the clothing. Probably better to do them one at a time to be sure.