In my experience, the better a show is doing at the box office, the harder it is to get rush tickets. Conversely, shows that aren't doing so hot are easier to get. As a backup, shows that aren't sold out often still have inexpensive seats left an hour or two before curtain, in case there aren't rush left. I got either rush or cheap tickets day of for It Shoulda Been You, Something Rotten!, The Visit and An American in Paris this season.