Elbereth

Elbereth is a goddess worshipped by elves. Engraving her name on a square prevents most monsters from attacking you while you are on that square. Engraving the E-word, as Elbereth is often referred to, is also a popular way to protect stashes. Note that there must be something, like you or a scroll, in the same square with Elbereth for it to work. No monster represented by an @ will respect the word, neither do any A, minotaurs or the Riders.

A fast method (writing in the dust or using a wand) is almost always preferable over a slow one (actually engraving).