The keyword STATES defines the dimension of the subspace used
in the diagonalization. STATES must be greater than or equal
to 
(for closed shell calculations), but it is strongly recommended
to allow for certain number of more or less empty bands (usually 10 to 20 percent).
Finally, ANDERSON MIXING influence the rate of convergence to self-consistency. Properly chosen the convergence can be very fast. Typically for bulk systems we use values between 0.2-0.5, smaller values being necessary for larger systems. For metallic surfaces, very small values are necessary (typically 0.03-0.05).
If using k-points, then it is usually a good idea (and this is done by default if using MONKHORST-PACK grids for k-points) to exploit symmetries. In this case, however, beware of including the POINT GROUP keyword to symmetrise properly the density. Finally, if starting from a high-symmetry structure, you may nevertheless want to use the full k-point mesh (apart from time-inversion symmetry related k-points), and in this case you can add the option FULL to the MONKHORST-PACK keyword.