Stop seatpost slipping

I just read an article in Mountainbike action.

Sidewalk chalk (or any chalk, I would imagine)

Put chalk on your seatpost* to prevent it from slipping.

Also, for those with bikes with rim brakes, apparently rubbing chalk on the braking surface prevents it from squealing. My MUni brake never squealed, but I assume it could.

*between the seatpost/seat tube interface…chalk elsewhere would have no effect, except temporary beautification