One very important thing is to use the AND operator when you search for more than two words.
Let's say you will search for topics that both containes the word "nucs" and the word "smoker". If you just write "smoker nucs" here now, you will get 158 topcs that containes "nucs" and/or "smoker" not only those who contain both. To search for topcs that have both you can type:nucs AND smoker
This will return 2 topcs which containes both words.
There is a flash-demo (movie) at phpbb.com about searching that might help:http://www.phpbb.com/support/tutorials/display.php?t=phpbb2_eu_search