Technically, there's a difference between BUMPing a thread and Necroing a thread.

BUMPING
This is specifically to the OP BUMPing his own thread, BUMP is actually an acronym for Bring Up My Post. This is against forum rules as the belief is if there is any merit to your thread, it would be discussed naturally by other members without having to bring it up to the top artificially. Bumping is only when the OP brings up the post, specially in a way as this:

Unacceptable Use
"Daily Bump" or "Any other thoughts?" or "Come on guys, what do yall think?" or "Why is everyone ignoring my thread?"

Acceptable Use
Replying to your own thread, after others have, adding relevant information to further the dialog, is not considered BUMPing. It's only when you are posting with no additional information for the sole purpose of bringing your post back to the first page.

Exemption
Daily bump is acceptable in Clan, Neighborhood and TaskForce Recruitment subforums



NECROING
Necroing may not be as cut and dry. Necroing is when you bring a post that has been long dead back to the front page.

Unacceptable Use

Necroing a post from three years ago just to say "Lol 10char" or otherwise not adding anything to the original conversation. Most of the time, conversations die for a reason. Many Necro threads I've seen that falls under this category is replying to a year old complain thread, say for example complaining about Zapping, just to say "Lol, this was fixed in the last update, derp!" as if that means anything to the two year old thread.

Acceptable Use
Using the search function, you find a post that may be a few months old but is relevant to a question or concern you have. You reply to said thread adding relevant discussion that would continue the conversation and get new perspective from newer users. How old is too old? That's like asking how hot is too hot for bath water. Each person would have their own tolerance level. It depends on the topic and how relevant the new information is being posted.