If it's fairly long I will get the general gist of the way the conversation is going, then look out for subsequent posts by the original poster (particularly if it is a problem they are seeking advice on), before I post. If it's short then I will read the whole thing - mainly because I'm nosey!
Just as an aside, proportion of threads lurked to threads posted on - probably post on one out of 30-40 that I read!
I agree with Miaou about working out how convo has gone and checking the original posters subsequent posts. Would have to agree on number I post on too, very low in comparison to those I read/lurk on !