Kristeen Hudson provides these 10 tips for successful preheader text on the Exact Target blog:

1. Use a call-to-action
2. Use positive wording 
3. Make your preheader text support you subject line
4. Keep in mind the length of the preheader text that will be displayed in the inbox
5. The marketing preheader text (aka the call to action) should be in the top left of the email
6. If you make the pre-header text too long it defeats the purpose.
7. Don't repeat the subject line as the preheader text
8. Preheader text is generally smaller then body copy, but it should still be readable
9. Write attention-grabbing preheader text
10. Try an A/B split with different pre-headers and test to see which emails get the most opens and conversions.  

Read the full post here: The ExactTarget Blog - 10 Tips for Successful Email Preheader Text .