At the heart of Usui Reiki are the Reiki Precepts or Reiki Principles. They are a set of 'rules to live by' that were an important part of the system taught by Mikao Usui. As you research Reiki for yourself you will see different variations and translations of this. However this is one which I follow :

Just for today, I will not worry
Just for today, I will not be angry
Just for today, I will give thanks for my many blessings
Just for today, I will do my work honestly
Just for today, I will be kind to every living thing