You might have switched it too slowly, which causes the fuse to blow because it's trying to power both high and low beams.. Or your switch might be not doing the full switch from high to low, and is leaving the high beam powered..
Not sure what you're running for a fuse, but I'd go with a 15amp..
A couple years ago I went with a relay light switching system from Eastern Beaver.com and it eliminates this.. Instead of putting power through the small wires and switch, it puts the power straight from the battery to the headlight, using the switch only to operate the relay, telling it to be in high or low beam..
The relay system give a MUCH brighter headlight, longer lasting bulbs, and eliminates the blown fuse if you make the switch too slowly.