Forget it on the Yamaha.

Having faced this in the past (and the same problem exists even more so for SMF's, where they might still be on the beat, but come in a whole bar early!), the only thing you can do is start singing REALLY loudly on the correct beat and hope that they listen to you.

And basically, if this is something that really occurs enough to worry you, drop the singer. No matter HOW good looking she is!