I always start w\ my trusty 6-in-1 screwdriver (link below). It handles pretty much any playfield screw, nut, or post in one handy package.
Titan Pinball's Little Box of Calm is also handy at preventing expletive laced tirades when you can't seem to find that nut\screw\washer that you dropped.
http://www.titanpinball.com/products_detail.php?ProductID=107