Choose a debatable topic. All thesis statements make an argument; they are not statements of fact. You should choose a position on an issue that someone else could reasonably argue against. For example: "Our university spends millions of dollars on our football program" is not debatable, as it is a statement of fact that can be verified. However, "Our university should cut the football budget in half and spend the extra money on academics" is a debatable thesis.
Narrow your focus. One of the most common problems that student writers have is making thesis statements too broad. Narrowing your thesis will make your argument more persuasive, and it will make it possible to fully develop your argument within the length requirements of the assignment. For example "Global warming should be stopped" is very broad, but "The US should require all cars to get at least 30 miles per gallon to prevent carbon emission" is manageable.
Use clear and direct language. Student writers overuse their thesaurus and try to use complex vocabulary to "sound smart." However, the best writing is clear, to the point, and uses a vocabulary that the writer is comfortable with.
Revise frequently. The thesis statement should usually be the first thing you write so you know where your paper is going, but you should come back to it frequently and revise. As your paper develops, your thesis will become clearer to you, and you will be able to express it more dynamically.