I want to second what Thogert posted about checking the direction of the bolt or nut to remove it. I think we've all been there!
When I am under the truck or contorted in some way to get to a fastener where the old rule "righty-tighty, lefty-loosey" is hard to apply, I use the Right-Hand-Rule.
The RHR is that if you curl your fingers on your right hand as if to make a fist, the direction your thumb points when pointing away from the hand (e.g. as if you were hitch-hiking) is the direction the bolt or nut will move when rotated the way your fingers have curled.
Of course this only works with fasteners that have right-hand threads. Those of you who work with pressurized gas cylinders know that for some specialty gases, the threads for the regulators have left-handed threads. Apparently in about 1970 Dodge thought it a good idea to use left-handed threads on some of their lug nuts/bolts! You have been warned! ;-)
Here is a picture:
Note this works for loosening or tightening.