Have your students practice write persuasive, argumentative thesis statements on prompts they enjoy (Peanuts, their favorite T.V. show, their favorite band, etc.) before they start writing their big essay.
This two page worksheet helps students practice writing single-point, argumentative thesis statements with a reason. You can adjust how you want students writing their thesis statements according to your needs, but I usually give them a very formulaic way to fill out the worksheet.
For example, "Lisa Simpson is the smartest character on The Simpsons due to the fact that/because she has a higher I.Q. than Homer."