Alcohol or other solvents like Acetone work, but another easy trick a lot of people aren’t aware of is to go over a permanent marker with a dry erase marker, and then wipe it off.

I shit you not, it works. We tested this across several whiteboards. First in inconspicuous spots, then on main parts of the board. Occasionally there was some residue remaining if the permanent marker stayed on for long, but the solvent they use in dry erase markers also erases permanent marker material.

YMMV on a tape measure though…