Try to reduce the number of clock signals generated or used. Another one is to try to get a "clean" clock signal, an external clock can sometimes be better than the internal one. Also the references onchip tend not to be the greatest so again an external reference might help. I once had to use a callibrator for this with an accuracy of 1/1,000,000 (although it might have been a little bit of overkill,1/200,000 probably would have done it!)