In 1 sentence, give the best piece of advice you can.

[yahhzoo]

Everyone makes mistakes.

[Lovescake]

" Don't Worry, DPS can be easily replaced "

[Recaldy]

Watch a video first.

[Fizzy]

Try playing all aspects of the game, not just one.

[Kazza2003]

Learn about encounters in advance if the information are available.

[Alistaire]

Avoid necrophilia.

[Erudain]

"drugs are bad mmmkay"

Now seriously:
"if you see something odd on the floor, it's most likely something bad will happen, so move out"

[Mcshiggs]

If at first you don't succeed, complain on the forums until it gets nerfed.

[RunaRugar]

Mostly if it's your first class in the game Listen to other people's advice.

[Codek]

Change your underwear daily!