Can you tell us some details about the application and why you need to improve the CMRR ?
One thing you could do is to use the op amp in the inverting amplifier configuration - both inputs are at ground potential (one connected, the other "forced" by the op amp) so the CMRR, however bad it is, doesn't matter. But I guess most of the time people are satisfied with the CMRR of modern op amps and don't worry too much about it. Maybe just get a better op amp ?