The site has a pretty powerful search tool, but I've also found that Google can be pretty persuasive in coaxing lost threads out of the aether.
Try using site:forum.barrowdowns.com as the first term, then whatever you're trying to search on. Note that terms in quotes will return that exact phrase -- this is what can make Google a little more powerful than the native search feature.
So for instance, if I wanted to find a particular post that I remember from a thread that was about Arwen, and Esty posted in it, and I remembered that somebody used the phrase "it will teach him wisdom" in the thread, I could search like this:
Code:
site:forum.barrowdowns.com arwen "estelyn telcontar" "it will teach him wisdom"
...and it will pull the exact thread I'm looking for. It's a good way to make a very narrow search if you remember some key phrase from an old thread.