The common wisdom you'll hear repeated ad nauseum is to "go and make valuable comments" on a blogger's posts to get their attention.
Cute, and it works, but there is a much better way.
Read the comments on any decent blog (and indeed forum) and they fall into three categories -
1. "Thanks" (i.e crappy one liners of no real value)
2. Me Me Me (commenter drivels on about themselves, yawn)
Blogs with lots of 1 & 2 have poor audiences. Blogs with a decent amount of 3 have something incredibly valuable - an engaged audience who aren't being fully served by the blogger.
This is your opportunity.
Instead of sitting there trying to come up with "something valuable" so say, go through the comments and answer every question other commenters ask, to the best of your abilities.
This does a few wonderful things -
1. Puts lots of you in the comments, and shows you know what you're talking about
2. Gains you new visitors/subscribers for near 100% of the people you help
3. Make the blogger love you because you've done their work for them - help their readers out.
Now go do it.