Anil Dash provides tips for pitching bloggers in the aptly named post Don't Be a Bad Pitcher!. Covering basic topics like, don't send unsolicited attachments, and put links high up in your email to grab the blogger's attention, this is a good basic post for anyone unsure of how to start pitching bloggers, or anyone needing a reminder of general good pitching practices.
Don't talk like a database. This one happens all the time. PR people are used to consulting their database of press contacts and skimming the profiles to see which ones should be targeted for a pitch. But the copy in those profiles is way wrong for bloggers, and results in emails that say stupid things like, "Dear Anil Dash, I know you write for Anil Dash blog covering the random horseshit beat, so this might be of interest to you..." Rewrite it in English before you send it, or just don't bother. It's only politeness that keeps me from publicly ridiculing the folks who send these.
This tip is particularly important when sending a pitch to any journalist or blogger. Personalize your pitch, and not just in a "fill in the blanks" kind of way. If you're sending off hundreds of pitches to different bloggers and journalists this becomes a challenge. Instead, I would suggest focusing your pitching on a smaller number of folks and write very targeted emails that speak to each individual. If you must send out a large number of pitches, taking the time to personalize each pitch, just a little more than Dash's example, will give you much stronger results.