I make your words my words...
Just wanted to add that getting off topic is absolutely normal (I'd say it's even more normal to drift than to keep sticked to the title...). However, as Violet said, it's not so good when you find a topic you would want to reply just to find out people in there are just discussing something very different and without anything related to the topic's title.
I'd say you could do the following: imagine you are reading a theory (for instance 'Dudley's Dementor'), and suddenly you remember something that would fit in there but is not quite connected with the title... Why not start a new topic, talking about what you read and what you remembered, and leave a hyperlink* in the 'Dudleys' Dementor' thread? It would prevent getting off topic, and we could still talk about different issues.
*
NOTE: To learn how to make an hyperlink, go
here.