You can help reduce the chances of a plug wire pulling loose from the spark plug terminal by adding shrink wrap over the end of the boot and the first inch or two of wire. Most manufacturers sell pre-cut shrink wrap for this purpose, but I use stuff my local speed shop sells for insulating battery cables because it is thick, works just as well and-most of all-it's cheap! At the bottom of this photo is a plug boot right out of the box. In the center is a boot with a short length of shrink wrap slid as far up over the boot as I could get it. And at the top is a boot and wire after the shrink wrap has been heated until it has shrunk into position.