If you're using the search bar that's on every page, keep in mind that it only searches within your current context. For example, in a thread, it will only search that thread. On a board, it will only search that board, etc.
I figured that out the first day
No, it's that it seems to draw on totally irrelevant data while ignoring exact text. Yet I can run the exact same search in google (from their site) and it will bring up the relevant text as the first result. I actually do this instead of using the internal search more often than not due to frustration. With custom search, it SHOULD do this faster and better than from the main google.com page.
As an example, I just typed in "my little bitcoin" (sans the quote marks) in both google and bitcointalk. This is a thread I started, and that is it's exact title. On google.com it's the first result. Here it's not even on the first four pages. (after which it times out with "you're searching too fast". )