A perfect sunset is a lot easier to shoot than you probably tell yourself it is because there isn't any one thing that defines what a perfect sunset is. A perfect sunset shot might not even have the sun in it. All you have to know is how to take advantage of the situation you're in at the time of your shoot.
Location: The first part of any sunset shoot is finding the right location. Fortunately, apps like Sun Surveyor can tell you exactly where and when to stand weeks ahead of time. What I like to do is find the area I am going to be shooting in, zoom out until I see about 10 miles around that location, see the direction the sun is going to set, and look for something interesting; a mountain range, jetty into the ocean, lighthouse, bridges, etc. Something in the foreground will always make for a more interesting sunset photo. Photos of the sun setting out at sea with nothing in the foreground are a dime a dozen and really aren't very interesting unless the clouds are very unique or the colors are filling the entire scene.
Date & Weather: Once I find a location, I look at the weather and golden hour times (Also available in Sun Surveyor). I love a good partly cloudy day (I'll explain below...) and I try to get to the location at least 30 minutes before the golden hour which is about 30 minutes before the sun sets so I can compose my shot. I was recently planning a sunset shoot underneath a famous bridge in Massachusetts, but when I got there I realized the canal and bridge in the distance made for a much more interesting shot so I ended up moving everything on to the bridge and shooting through the bars.
Clouds: Clouds will make or break your entire composition. I can't tell you how often people think clear days are the best days to go out shooting. In my opinion, clouds are one of the most beautiful parts of a sunset photo; Clouds help your composition, add depth, add a sense of movement in a long exposure, and enhance every color given off by the setting sun.
Questions & Answers:
What if the sky is clear or the clouds are scarce? In situations like that, I try to make the shot less about the sunset by including more in the foreground. If you're taking a shot of the sun setting out in the ocean try adding part of the beach or a jetty to your shot. If you're shooting from up high, try tilting your camera down a bit to include more of what's happening below you. If you're shooting the sun setting behind an object (mountain, trees, lighthouse, etc.) wait until the sun is half covered and lens flares are taking over your shot. Lastly, If there's nothing interesting to include in your shot consider changing locations.
What if the sky is cloudy and completely covered? I always like to think there's a possibility that the sun will break through at the last minute, but that's not always the case. If the sun isn't going to break through and the clouds are too thick to let the golden hour colors through, practice your composition and come back. I am not saying you can't get a good photo in general, but it wouldn't be a sunset photo which is what this post is all about.
How do I make my sunset more fantasy-like? I love seeing shots that are more dream-like than a quick capture. This can be done in a number of ways, but my go-to method is using an ND filter to extend my shutter speed upwards of 20 - 30 seconds. By doing this any moving water will be smooth and clouds will look like they are in motion. Other methods include using a tilt-shift lens to "shrink" the scene, partly blocking the sun to increase lens flares, or blocking most of the sun to add an orange glow to the scene.
Where do I put the sun in my shot? That's a hard question to answer. It really depends on what's in your shot. There are all kinds of "rules" when it comes to composition like the rule of thirds and golden ratio, but the sun's position will be entirely based on the scene. Try to think of your camera's rule of thirds grid as a flexible helper. It's mainly there to help you fill the scene so you don't end up with areas that are too empty while others are overly loaded. I look at these "rules" as a great starting point because over time you will start composing your shots better without their help.
I hope that helps! Any questions or comments? Let me know below.