The following "best practices" are designed to have an immediate and noticeable impact on your campaign performance.
Step 1: Keep the Pre-Header Text Short
You are working with limited space in the inbox, so like the subject line, less is more in this instance. Due to the fact the # of characters supported varies with each email client try to keep it under 70 characters to be safe.
Additional support for brevity can be seen in the differences across email clients (particularly mobile email clients):
Pre-header text in the Gmail app displays right after the subject line.
iPhone 6 Plus Mail app renders up to three lines of pre-header text.
Windows phones only show 40 characters before cutting off the message.
Step 2: Treat Pre-Header Text as an "Extended Subject Line."
Be specific and to the point. Emphasize the call-to-action (or secondary CTA) and focus on additional compelling information. Sum up the email, but avoid being repetitive.
Step 3: Pack In Those Strategic Keywords!
Keep character limitations in mind, and pack the beginning of your preview text with keywords and phrases that perform. You'll want the"good stuff" to show up in clients that may cut characters.
Some companies like to include a unique CTA here that compels the reader to open and engage the content of the email.
Step 4: Create A/B Test Versions of Your Pre-header in Addition to Your Subject Line.
I cannot emphasize how important subject-line testing is, but an often overlooked area of opportunity resides in the ability to test pre-headers as well. This could make that "extra" bit of difference -- compelling your subscribers to open the email, and increasing the overall reach of your campaign.