Backcountry has it right. If you have base burn, you'll need a base grind. This is when the base oxidizes and it will no longer hold wax.
If they're not burned and just really dry, just keep waxing them. I'd treat them like new skis, so go through 5 or more wax/scrape cycles with a cheap warm temp wax before they ever touch the snow, then finish with something closer to the temp you want or an all-weather wax. Rewax often after that - they'll just keep getting better.