• summarizing the controlling idea in a paragraph 
• link one paragraph to the next
• may qualify the information
• don’t repeat the topic sentence, but they re-enforce it and relate it to the topic of the following paragraph. 

Example:  

Therefore, the reason bicycle helmet should be mandatory is because of their vital role in improving road safety and accident survival rates.

(This sentence clearly restates the topic sentence, but does not give the reader any idea of what topic will be covered in the next paragraph.)

As seen, bicycle helmets have been proven to improve road safety and accident survival rates yet the number of people who actually wear bicycle helmets remains relatively low.

(This sentence reinforces the fact that bicycle helmets are important to road safety and accident survival rates; it also introduces the idea that the number of people who wear helmets is relatively low; the next paragraph in the essay would be about how many people actually wear bicycle helmets.)

Resources to help develop effective Concluding Sentences

IELTS Share (4.09 minutes) 

  Meg Mosier (10.04 minutes)
- thorough discussion with examples; highly recommended