I like using a Pentel .9 mm mechanical pencil. I use the rolling technique to keep it sharp. For most stuff it is thin enough with our sharpening but on precise stuff I will take one pass with a piece of sandpaper. It is thick enough that it doesn't break too often, but being a mechanical it's just a few clicks from back to working order. You can find it at amazon.com, Office Depot, or a big box store. They run about $10 for a 2 pack. I use this or a carpenters pencil sharpened with a block plane for a super fine line.