Remember that your thesis statement is meant to be a tool for your reader. It should clearly spell out what the bulk of the paper is trying to say. A reader should not only be able to determine the meaning of a paper from reading the thesis, but he should be able to refer to the thesis statement and see exactly how even the most abstract content in your paper ties into it. You should also remember that a good thesis statement should be a tool for yourself as you are writing your paper. Referring to your thesis will keep you from going off topic in the paper. If you think what you just wrote doesn't fit with what you are trying to say, either rewrite it or remove it from your final draft.
Keep the thesis statement simple to understand. The thesis statement is the essence of your paper boiled down to a single sentence. It should not be difficult to understand. If it sounds too simple to you remember that you have several pages to write that will elaborate on it. A good rule of thumb is to go back and read your thesis statement as you write or rewrite your paper. If it's easy to understand while still capturing the essence of what you are trying to say with the rest of your paper, it's a good statement. If it comes across as too abstract or difficult to understand, you may want to consider coming up with something simpler.
Go to the professor for advice, visit a writer's workshop, or ask a classmate to read and critique your paper. Do not be afraid to ask for help. All of these can be invaluable resources for a writer at any skill level. No matter how much you know, or think you know, about writing you can always learn more. A second pair of eyes may just find something in your paper that you overlooked.