In the net there are some sites that offers to sent for you 100 or more comments in as much blog behind payment. Well, that's absolutely unnecessary and in most case even called spam, because you don't now in advance what type of comments will be posted under your domain and site. If you have a blog probably have already experienced people that post in your blog a sort of message just to obtain a link back to their site. But, instead, partecipate constantly in another blog, posting opinions and not pointless sentence, can't be absolutely called spam.
Ok, so, find a blog pertinent to your web space, or your ideas, and write a comment there. And in this matter it's very useful this link: commenthunt.com - a search engine for blog without nofollow tag (we already talked about nofollow, here a link to wikipedia).