How do you even find all these old things? I find the forum's search function to be sort of inadequate.
The forum search is terrible, but you can get a complete list of a poster's topics and posts by looking at their profile. So if you remember who started the thread you're looking for, you can find the thread by looking through that person's topics.
If you don't remember who started the thread but you do remember someone who posted in it, you can look through that person's posts. But that'll take a long time even with control-F unless it's a low-output poster or you know roughly when they posted in the thread. So I don't really recommend that method.
If you're forced to use the search function, the key is to search for as many of the words that are in the thread as possible. If you're looking for a thread about Echoes Of The Beast that I posted in, you can search
echoes beast sanctaphrax
and you might get the thread you want. Or you might not. It's easier if you remember a distinctive word or phrase that won't be found much outside of the thread.
Usually finding anything with the forum search function takes a few tries, so it's good to have a large-ish set of keywords that you can mix and match.
(Using Google is much easier, but many of the same methods apply. Unfortunately Google doesn't work on this forum.)