If you have no a priori knowledge of your system, for instance, if you do not know whether your system is an insulator, semiconductor or metal then always use Gaussian smearing ISMEAR=0 in combination with a small SIGMA=0.03-0.05.

This is not the default in VASP yet, so to be on the safe side, you might want to include this setting in all your INCAR files.

For semiconductors or insulators, use the tetrahedron method (ISMEAR=-5), if the cell is too large (or if you use only a single or two k points) use ISMEAR=0 in combination with a small SIGMA=0.03-0.05.

For relaxations in metals, use ISMEAR=1 or ISMEAR=2 and an appropriate SIGMA value (the entropy term should be less than 1 meV per atom). For metals a reasonable value is often SIGMA= 0.2 (which is the default).

Mind again: Avoid using ISMEAR>0 for semiconductors and insulators since it might cause severe problems.

For the calculations of the DOS and very accurate total-energy calculations (no relaxation in metals), use the tetrahedron method (ISMEAR=-5).