When you're not around to identify your art (and sooner or later you won't be), your signature identifies it for you.
Unfortunately, far too many artists treat signing their art as little more than an afterthought or inconsequential incident, like signing a check or a credit card receipt, like putting your name on it hardly even matters.
Maybe they think everyone will continue to know who they are for all eternity. So rule number two is sign your name clearly enough for anyone to read. You don't necessarily have to sign legibly on the front of the art, but make sure you clearly sign or otherwise label or identify your art as being by you somewhere, anywhere-- as long as it's directly on or attached to the art.
No matter what your signature looks like, what form it takes or where you put it, no work of your art is complete without one.
Your signature identifies your art for all time as having been created, completed and approved of by you and you alone (with the exception of collaborative works, of course).