Include a small picture of yourself in the message window and make your visitors feel connected to you (people remember human faces more than any other type of picture). Also, include a short, personal message. People like buying from people they know, like and trust.
Check out the WP Stealth Note DEMO BLOG. You can customize all the content, colors and placement of the window.
Link to a page on your blog that reviews a product. Include an opt-inform in the message window, and ask your visitors to give you their email in exchange for something cool that they will find valuable.
Create a YouTube video and include it in the message. It’s a well-documented fact that videos convert much better than simple text.