Use it as a signposting opportunity
If your introduction and conclusion are clear enough, it should direct the reader through the main body easily.
Avoid being repetitive
Whilst an introduction and conclusion cover similar areas, they are not the same. They both serve different purposes; therefore, they require their own attention.
10% of your word count
Unless you have been given a specific word count for your introduction or conclusion, each section should only be 10% of your word count (20% in total). The remaining 80% of the word count should be for your main body.
Avoid going into too much detail
You do not want to take anything away from your main body, where you will get the majority of your marks.
Make sure that you are only mentioning relevant points
If you are writing five hundred words in your introduction and your essay is 1000 words, then you may be going into too much detail and including irrelevant information.