You gave your prospect something when they originally signed up with you. It may have been a special report on your topic, an eBook, an audio recording, some tips, or any of a varity of other things. Even if you gave them helpful tips, give them some more.
Set up a series of 10 autoresponder messages to go out over a 20 to thirty day period. This means they will receive a new tip every 2 to 3 days. Number them, and make sure they are helpful and informative so your prospect will be able to put them into use right away.
By numbering them, people will look forward to receiving each one in sequence. End each message with an action step or call to action, such as asking them to comment on your blog or visit a specific site. Many people will save these tips in a folder on their computer, or even print them out. Never underestimate the power of the information you are providing to others.