I am not so much worried about the inevitable spammers, but I would like to see more comments from people with something worthwhile to say. How can we entice them to talk?
Something no-one else seems to have picked up on is the quality of the blog postings themselves. Are they about topics anyone cares about? Are they well written, do they provoke a response?
Making posts on other blogs is a very good idea as @Osvaldo states. If you want people to come and post good comments on your blog, you going out and doing the same on other peoples blogs, helping add value to their blog is likely to reap rewards. Either in terms of the blogger themselves returning the favour, or attracting readers of said blog who enjoy your writing style/opinion.
Failing that, run a competition. Offer a 5/10 dollar amazon voucher for the best posting in a month.
If it's a blog on tech subjects (web/internet etc) you could try removing the
rel="nofollow" from link to the website of the commentor and make it loud and clear on your blog that you don't use nofollow for good interesting comments.
I would be more happy to spend time adding nice and interesting comments on blogs that reward me by adding a link to my website rather than on blogs that reward me nothing!
Obviously this means that you need to review properly each single comment before approval and remove the link to the website if the comment is poor/useless/simplicistic like and idious: "Cool article, thanks for sharing!" (and obviously if the comment is just spam)
But reviewing comments should be the normal job of whoever runs a blog, the
rel="nofollow" was needed because:
too many bloggers are lazy and approve any type of comments without even reading it
many non-tech guys bloggers do not know that they were giving reputation away (see it as PR) to commentors' websites by providing them a free link to their websites.
some blogs have so many followers that would be impossible to look into each comment, but still the choice of the editor could be to publish only the comment he can read and not all of them.