There's been some talk on here lately about bolts working loose so I thought I'd pitch this stuff again. It's cheap and provides a quick visual check to see if a bolt head or nut has turned because it's starting to come loose. Of course, it only works on fasteners you can see:
http://www.aircraftspruce.com/catalog/cspages/f900.php?clickkey=3836
It comes in different colors but I use the yellow:
http://www.aircraftspruce.com/catalog/cspages/f900.php?clickkey=3836
It comes in different colors but I use the yellow: