Updated: Sep 2, 2018
This is a very good question. Personal statements must strike the balance between blowing your trumpet and not sounding proud.
One of the easiest ways to achieve this is by acknowledging those who were instrumental to your success. For example, if you led a team to record significant success, you should acknowledge that the success was recorded through the dedication of the team and not just your hard work.
If you received mentorship of any form, you can also mention this. One thing you can also do is to send your essay to a friend you trust. This friend will help you confirm that while you are not overselling yourself, you are also not under-selling yourself.
Now go and write that essay!