1. THESIS statements Say what you mean and mean what you say
p.s. This was all stolen from Harvard’s Writing Center

2. After reading your thesis statement, the reader should think, "This essay is going to try to convince me of something. I'm not convinced yet, but I'm interested to see how I might be."
The Job

3. A thesis is not: A Topic A Fact An Opinion
"Reasons for the fall of communism" is a topic.
"Communism collapsed in Eastern Europe" is a fact known by educated people.
"The fall of communism is the best thing that ever happened in Europe" is an opinion.
A thesis is not: A Topic A Fact An Opinion

4. It should tell what you plan to argue, and it should "telegraph" how you plan to argue—that is, what particular support for your claim is going where in your essay.
In Two Parts:

5. Let’s look at the steps 1) Read and analyze your sources
2) Write a working thesis
3) Keep your thesis PROMINENT in your introduction
4) Anticipate the counterarguments
Let’s look at the steps

6. A thesis is never a question
Readers of academic essays expect to have questions discussed, explored, or even answered. A question ("Why did communism collapse in Eastern Europe?") is not an argument, and without an argument, a thesis is dead in the water.
A thesis is never a question

7. "For political, economic, social and cultural reasons, communism collapsed in Eastern Europe" does a good job of "telegraphing" the reader what to expect in the essay.
However, political, economic, social and cultural reasons are pretty much the only possible reasons why communism could collapse. This sentence lacks tension and doesn't advance an argument. Everyone knows that politics, economics, and culture are important.
A thesis is never a list

8. A thesis is not vague, confrontational, or combative
An ineffective thesis would be, "Communism collapsed in Eastern Europe because communism is evil." This is hard to argue (evil from whose perspective? what does evil mean?) and it is likely to mark you as moralistic and judgmental rather than rational and thorough.
It also may spark a defensive reaction from readers sympathetic to communism. If readers strongly disagree with you right off the bat, they may stop reading.
A thesis is not vague, confrontational, or combative

9. A thesis IS…definable and arguable
"While cultural forces contributed to the collapse of communism in Eastern Europe, the disintegration of economies played the key role in driving its decline" is an effective thesis sentence that "telegraphs," so that the reader expects the essay to have a section about cultural forces and another about the disintegration of economies.
This thesis makes a definite, arguable claim: that the disintegration of economies played a more important role than cultural forces in defeating communism in Eastern Europe.
A thesis IS…definable and arguable

10. A Few Student Examples

11. Ultimately, the Stonewall Riots affected the society and politics of American history as it was the symbolic movement that strengthened gay empowerment and kick started the fight for LGBT rights in the United States. 
What? How? Why?

12. Prohibition instigated the development of organized crime by providing Americans the opportunity to be the primary dealer of illegal substances through the exploitation of corruption and violence.
What? How? Why?

13. With the rise of the hippie subculture, new lifestyles were introduced that opposed the dominant culture, thereby initiating non traditional American values.
What? How? Why?

14. Several influential inventions and innovations came about as byproducts of the rivalry between the inventors of the direct and alternating currents.  
What? How? Why?

15. Therefore, the Chicano Movement strengthened the Mexican American community and created a new identity that they would be proud of and claim as their own.
What? How? Why?