I've never been particularly good at mental math. I think I'm too visual. But for those of you who are looking for a good resource, check this out:
One problem given to a fourth grade boy at the 2002 Mental Math Competition in Miami, Florida, went something like this, "Nine times nine, plus 19, times four, divided by 25, square root, add six, multiply by ounces in a pound, minus 60, plus 44, square root." "Twelve!" shouted the youngster, of Ben Sheppard Elementary school, which won the 2002 championship in the competition, enjoying a sound victory over even North Miami Senior High.
Find Project Motivational Math products at http://www.atlasbooks.com/marktplc/00814.htm