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.

Elbereth stops working when it degrades into a different word. If you are writing in the dust ([E] [-]), you might want to write it several times. The immediate result of one test was "ElberLth ElbereGh ElVereth Ylbereth Elbeheth Elb;reth Elbereth Elbereth tlbere#h".

Engrave Elbereth quickly

A fast method (writing in the dust or using a wand) is almost always preferable over a slow one (actually engraving). Thus it is normally better to write with your fingers, which is fast, but in the dust, than with a hard gem or with a ring with a hard gem (such as a ruby ring), which is slow.

This is because the slow methods give monsters time to kill you. This is a problem both when you need "Elbereth" quickly or if a monster surprises you. Note that characters reading a spellbook or eating a food ration will stop when approached by a hostile monsters, but characters busy engraving will insist on continuing the job. Thus any character engraving with a hard gem or other slow method is vulnerable to attacks from monsters.

For a permanent Elbereth, use a wand of fire or wand of lightning to burn it. The wand of lightning can blind you, but that is an acceptable risk during an emergency. Otherwise, unless you can easily cure blindness or protect your eyes, avoid any engraving with the wand of lightning.

NetHack brass

A warning to players of the NetHack brass variant: Avoid engraving Elbereth with wands. While vanilla uses 1 wand charge per engraving, NetHack brass can consume extra charges. An engraving with a wand of fire, for example, costs the usual 1 charge engraving with a wand, an extra 1 charge because the wand is a wand of fire, plus 1 charge for each letter engraved. To burn Elbereth successfully, you would need 10 charges in your wand of fire; you probably do not have them.