Well, I've been thinking about it, and it's not too difficult to try and stay on subject. It's nature for people to want to talk with other people, and sometime it's difficult to control that urge. I think that it's a tad too harsh to expect everyone to try and stay on topic, but we need to get better at recognizing when we're becoming off topic or derailing something, and try to stop that from going any further.
Something interesting that was brought to my attention by the best website in the history of the universe,
Cracked, was that a common tactic from people on the internet is to try and tell their stories to people that simply don't want to hear them. I think this is related to the issue we're experiencing here, in that someone wants to hear what this other person has to say, but no one else does. In that situation, you should take a step back and try to decide whether the conversation would be better done in a Private Message, or something else. I hate to beat a dead horse, but another situation that I seem to see a lot on the forums I visit, including this one, is people making uncannily specific topics to tell stories that people may or may not want to hear. This can be expressed in many different ways, from a humorous story about one's pet chinchilla, to a question about one's homework relating to the elasticity of an elongated and oversized cannula comprised of rubber when subjected to undue pressure from a large phallus.
Such posts and question would be better served in less specific conditions; for the former example, a topic that is general in topic, such as a topic about funny stories involving pets, or even a topic about funny stories would be the perfect place for such a verbiage. For the latter example, a thread that people can simply post their questions involving interests or other philosophical matters would be quite useful.
Hope that that wasn't too much of a strenuous read for anyone... It is quite informative, particularly the section beginning at word 238.