Layout and Circuit.
Use input diff. pair with large area MOSTs operating in weak or moderate inversion region. Use well matched load of diff. pair, proper sizing and strong inversion for current mirror.
One of source of systematic offset is difference of drain-source voltages of input diff. pair. U can eliminate this with proper biasing second stage or another technique which's related with CMRR and PSRR improvement technique in publications.
Use well matched layout (keywords: common centroid, dummy structures, placement, length of connections, metal coverage, temperature and machanical gradients and so on).