Find blogs that talk about the same topic as that of your startup's and post comments that link back to your startup, helping your potential customers find their way to you.
Remember to avoid spamming. Readers will easily see through your spam and will not visit your website. Make sure your service does provide something of value to the audience related to the article you are commenting on.
Good ways to find blogs is to use Technorati, or is to use the Google operator "blog:keyword" to find blogs that are of interest.