• When your email subscribers look at their inbox they are deciding whether or not to open your email.  The subscriber is using the preheader text is used as a prescreening tool to determine whether or not to open it.  
• The subject line is your attention grabber, but the preheader text allows for more context of what the email is about. 
• Without preheader text, it will take the body text of the email and use that text were the preheader should be. 
• The preheader text should not be longer than a sentence that summarizes what the email is about.

• Make sure to focus on the key content in the first portion of the preheader text.
• Even though some might only display 30-40 characters, web devices like Gmail display around 100 characters, including spaces.  Take advantage of all the available character space.
• Include a call to action to further emphasize the customer to open the email in their inbox.  
• Be careful of any words that can be flagged as spam.
• Try to personalize the messaging. 

According to Litmus, not all email apps support preheader text.  Below is a list provided by Litmus showing which email apps support preheader text.  (Source: Litmus)