I agree with Richard. Maybe in the past when things were expensive and in daily need then it was worth drying them out and trying to save them by waxing them, etc., but you can buy two big boxes of cooks matches for £1 these days.
It's not worth the hassle, and if you are needing a match now, then you need to know it's going to work.
If the heads are already crumbling, chances are they'll fall apart when you try to use them regardless of what you do.
Wax new ones if you're going to.
M