The last thing you want is metal filings from previous tunings embedded in your wax. And wash your hands so you don't wax in your own dirt and oils.
Now, to prep your skis, stretch rubber bands around your binding's brake levers to get them out of the way. Next, run a brass brush aggressively tip-to-tail along the base to work out any surface grit. Then do the same with a beefy fiber pad to remove any tiny spurs you may have stirred up. If it's been a while since your last wax, you might have to do a clean wax, or hot scrape, to get out the gunk (click here for hot scrape tips). Finally, run masking tape along the sides of the skis to keep wax from dripping into your bindings.