Another thing that can be done is to read through an old thread picking out aspects one finds interesting, worth further discussion. Then, start a new thread, with the topic stated in one's own terms. Optionally, reference the older thread with a link.
The benefit here is that the current forum participants don't need to read through that old thread (unless they wish), but can respond to the new presentation. Once in a while a necro-ed thread does gain a new life, but more often it does not, because it's the author of the necro post who's interested and others coming to that post aren't similarly invested. But a new post gets more responses because it's more immediate.