For starters, to write a good artist statement you must know what your work is about. At times you need a very brief statement which is often referred to as an elevator statement. This means it is something you could say to sell your work in the time it takes to ride a few floors on an elevator or in a few sentences. Then there is the typically artist statement which is a few paragraphs or sometimes just one. From there you may need an extended artist statement, but probably only if you are in graduate school.
I personally like to have at least three paragraphs in my artist statements, however, others do it differently. However, I have read a few really nice two paragraph artist statements. Some of the things you may want to include in your artist statement are: why you created the work, what the idea behind the work is (however, some people like to leave their work totally up to interpretation and say nothing about the idea behind the work), whether the project is still ongoing or finished and other artists who have influenced you. Keep in mind all of these things could or not be included along with other details. What is important to communicate in words about your work may not be important for others.
Most artist statements are straight forward; however, you set a tone in your statement. The tone could be humorous, antagonistic, or political. If you read my artist statement from graduate school, you will notice that it reads like a research paper, this is because the work was highly research based. If you look at my art website betsywilliamsonphoto.com you can see other artist statements that I have written they range from one paragraph to four paragraphs and also simply a definition.
Keep in mind, even though an artist statement is a bit of a personal statement you may want to incorporate a quote by another photographer/artist or make a connection to an art movement or artist influencing the work.