Put a little thought into which tie knot you are using, guys!
Different techniques for tying go with different types of collar, which in turn go with different types of face. The general rule is that thin faced fellows should wear a wide-spread collar, and vice versa. In keeping with that, use a wider tie-knot (i.e; Full Windsor) for a wide collar, and a smaller knot (Four-in-Hand a.k.a. the first knot pretty much everyone learns a.k.a. the one Shane described upthread or a Half-Windsor) for a narrower spread. It's also worth noting that wider/thicker/more ornate tie-knots are more formal than thinner and simpler ones. In other words, you wouldn't use a full Windsor knot underneath a sweater or with a sports jacket/blazer and chinos/jeans. I personally favour the half-Windsor over the four-in-hand most of the time because the four-in-hand actually produces an asymmetrical knot, which doesn't look as neat or put-together as other symmetrical knots.
Here's the two resources I head straight to when I wanna try a new knot or when I need refreshing on the old standards:
How to Tie a Tie- Ben Silver Contains a few interesting, more obscure tie-knots, as well as bow-ties and ascots(!)
How to Tie a Tie- The Art of Manliness Features Youtube videos for all the major knots, plus the mysterious and enigmatic Shelby knot.