In some cases it makes sense to start from an old WAVECAR file (for instance to continue relaxation or to continue with an increased energy cutoff ENCUT). In this case just keep the WAVECAR file and start VASP. Again, an empty INCAR file will suffice. The defaults are now:
ISTART = 1 # continue from WAVECAR file ICHARG = 0 # charge from orbitals NELM = 40 # maximum of 40 electronic steps NELMIN = 2 # minimum of two steps NELMDL = 0 # immediately update chargeYou can set ICHARG=1 by hand if an old CHGCAR file exists. If the charge sloshing is significant this will save a few steps, compared to the default setting. To continue relaxation from a previous run copy the CONTCAR file to POSCAR.