Collar stays--those little plastic pointers that come in a baggie zip-tied to your dress shirt's third button--are mandatory unless you are wearing a collar that secures itself (chiefly a button-down collar or a tab collar). They force your collar to fit the way it's supposed to, they properly frame your tie knot if you're wearing one, and they provide a bit of structured elegance to an open collar.
I bend mine a bit (as though they were an arc of a circle about 18" in diameter) to give my spread collars a little bit more of the British look. This also works well on point collars, but only if you're not wearing a tie. If you're wearing a tie with a point collar, stick with unbent stays.
Keep a couple pair in your desk at the office in case you forget to put them in at home. And be sure to remove them before you take the shirt to the cleaners.
You can pick up a bunch for cheap over at Jos. A. Bank.