MIRRORS
"How do you shave?" - Reptile Boy
Mirrors can not kill vampires other any other monster, but if you break them
you get seven years bad luck and think how bad that would be over the Hellmouth!
You can use mirrors to spot vampires though; they have no reflection, (eg reflect-me-not
Angel in 'Reptile Boy' and the picture above from 'What's My Line?')
HOW TO USE A MIRROR
Well, you just look in it, don't you. Not difficult, and they're so easy and portable to carry around. If you don't have a mirror though glass or any other reflective surface works just as well.
OTHER USES FOR A MIRROR
Use lots of mirrors to create a bulky (and complicated) vampire trap (see the weapon 'Sunlight')