A couple of other ways to find things:
Click on your own name, then on "show posts" to find threads you've commented on.
If you know whose post you're looking for, find their name, click on it and scroll through their posts.
Neither of these methods are foolproof, just other things to try.
Also, I think it helps if you can search within the correct topic. Don't look for something that was in "Taxes" from "Ask A Mustachian". Although, if you can't find something, how the hell are you supposed to know what topic to search within?
I think MMM never expected the Forum, or the Blog for that matter, to become the monsters they have. Lack of searchability underscores that feeling. A little better organization would go a long way, but, hey, it's a free site, right? I'm thankful for what there is.