I thought I'd dig out my Virtual Boy a few days ago. I thought the kids might be interested to have a go on something a bit different to the Wii. Unfortunately, one of the displays had started playing up which spoilt the effect somewhat. A quick bit of research latter and it turns out that the trick is to take the display out, stick it in the oven for a couple of minutes, and then rub your finger on it for a bit. Book.
Except getting the thing open is a bit tricky. The security screws holding the unit together need something called a gamebit. I found one on ebay and got ready to go, but some of the screws are so deeply set that the gamebit wouldn't reach them! So I had to dremel the hex section to make it circular and cut a slot in the end for a screwdriver. Eventually the case was opened and the offending display was removed. Two minutes in the oven later, a bit of rubbing and we're back in business!
The kid's interest lasted about 30 seconds.