This is more of a pressure thing than a mental game, but here it is...
At the end of practice we will, on ocassion, play "21". There are 21 outs in a game, so we'll put the defense in place, then have coaches hit a variation of ground balls, line drives and fly balls, with baserunners running from the opposite box. The object is to get to 21 without any errors. Once 3 outs are recorded, the bases are cleared. If an error is made......yep, restart the count. It's a simple game, but the pressure the players put on each other (especially from about 15 outs) gets pretty intense. If the defense is solid, we're out of there in 15-20 minutes. If errors are made, it can get ugly....but it's also a great way to create in-game tension and see who can deal with it.