When it comes to creating a really great sunset photo, timing is everything. While sunsets are always pretty to watch and photograph, often being really careful with your timing will turn a good photograph into a really nice one. I prefer to shoot the sun just a few seconds before it hits the horizon because there is this added element of the sun about to disappear. So, while I will usually shoot the sunset once it begins to color the sky, I reserve a few frames for the moment when the sun is about to hit, or is just hitting, the horizon.
The funny thing is that the closer the sun gets to the horizon, the faster it appears to be setting--like all of a sudden it's in a hurry to hide. It's really important then to be sure you and your camera are ready for those last few seconds. Also, sometimes there are other objects in your composition that you have to time along with the sun. In this shot of a sailboat moving past the sunset, I wanted the sun at the horizon but, of course, wanted to catch the boat in the frame too. I kept hoping the boat would slow down because it was moving pretty quickly and the sun wasn't yet in position! Fortunately I got off this one frame with both exactly where I wanted them. If that boat had been moving a tiny bit faster though, it would have left the frame too soon.
By the way, I also look for what artists and photographers call "points of tension" in situations like this. The little gap between the hill and the horizon is a point of tension because the mind knows that something is about to happen there, even though this is just a still photo. The gap between the bow of the boat and the edge of the frame is a larger, more subtle point of tension.