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Yeah, these problems can be very hard to find, especially when the ground only appears when vibration brings it in contact with a metal part. A volt/ohm meter can be useful, but often isn't.
Does it happen irregularly, or after the same amount of time since the fuse blew last? Is it more frequent in hot weather?
A fuse can carry just a little too much current for a while before it blows. It blows when it's too hot (hopefully from the current passing through it, but it's possible that very hot weather can cause the fuse to blow sooner) . If there doesn't seem to be any rhyme or reason to the fault, it's probably an intermittant ground short circuiting the "hot" wire to ground, which will blow the fuse with a quickness and a little 'pop'. Sometimes looking at the fuse can show the difference between a sudden blow vice a gradual melt - if you have black marks on the inside of the plastic window, you probably have a intermittant ground.
If you suspect a slight overcurrent and have a battery charger, you might try leaving the ignition on (so the headlight is lit) and the engine off for a while and see if the fuse blows. Not so long that the headlight becomes dim, or you may damage the battery. Don't leave the battery discharged overnight or longer, or it might stay that way. If the fuse blows with no vibration, you might have a problem with one of the components supplied by that fuse drawing too much current. This type of fault is not likely at all, but it's easy to check for.
If it indicates a intermittant fault (much more likely), you might try wiggling the suspect wires with the headlight on to try to simulate the vibration causing the problem. This can be hard to do, because many spots (like inside the headlight nacelle) are inaccessable to wiggle.
When I get a persistant intermittant ground like this, I figure out (from wiring diagrams) where the fault could possibly be, then clean the wires carefully with isopropyl alcohol (no residue) or WD-40 (doesn't harm wires or conduct electricity) and then inspect them under a *bright* light. Look for shinyness (copper showing) and for wear scars in insulation. A .1mm cut in the insulation might not be visible under normal garage lighting. I know that this isn't always possible without major dissassembly and cutting of insulation tubing, so look for the problem in the most accessable and likely spots first. An effective repair can be as simple a matter as wrapping a few layers of electrical tape around the cut, but try not to do this if the wires are likely to get wet or dirty - electrical tape adhesive can attract dust and bring the problem back.
Hope this wasn't too long and that it was helpful.
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