JohnT, thanks. I think you only need to start a completely new thread if there isn't already a recent one on whatever puzzle it is you wish to discuss. It used to get very confusing when there were three, four or five threads on a Times Cryptic, say, with lots of duplicated pleas for help, answers etc.
To contribute to an existing thread, you simply 'reply'. Many people add 'John, at post 19' and then go on to comment - that makes it easier to navigate through the thread (and avoids the slight messiness if three other replies have come in between-times!)