Spammers often send out mass comments to multiple sites with links back to their site. Instead of choosing random places to leave comments, you can turn this into a white hat method by selecting forums or blog posts that are complementary to your site and leaving thoughtful comments.
Whatever online strategy you employ, see to it that your SEO provider sticks to ethical techniques. Otherwise, you risk getting your website blacklisted by major search engines. A reputable SEO consultant from a company like Whitehat SEO Ltd. can help you stay on the right path.