I have been on message boards in various industries for five years now. I was doing posting for pay. It was easy for me because I was on so many forums anyway and figured why not. The extra money helped with my latte addiction
Some tips from my experience if you are going to pay someone to post for you.
* Give them the username and password that you want them to use. Or at least have them tell you what it is. That way should they quit working for you or other problems arise you still have access to the posts/links if needed.
* Have them send you a report letting you know where they posted. Be sure and tell them any forums you may *not* want them posting on.
* Have them start with 10 or less posts at first so they don't appear *spammy* then have them go back and reply to new posts.
* Whenever possible try to pick a username with the letter A or B - closest to the front of the alphabet.
I think thats it. You can also hire people to post real replies on blogs. Most of mine were never deleted because I made sure to be helpful not spammy. Most blog owners will leave a keyword name in place if the post is relevant and still have them sign the body of the email with a name.
Like anything else don't do anything in excess, moderation is best.