Because writing the email body every time you set up a report for a client takes lots of time, we have Email Templates.
So basically, just write a standard body text for your emails and save it as a template. The templates can be selected when you are about to send the report or when you schedule a report.
Manage your Email Templates
Go to the Email Settings page, you'll find it clicking on the Account's name (right top part of the page).
Click on "Add a New Email Template":
A pop-up will show where you can write the standard message. Yes, you can create more than one email template:
Save the template and you are good to go!