I use a solvent for this purpose and let it nearly dry then a razor blade to scrape off the black. I hate doing it so only do it once a week!
Not sure if there is a way of keeping it clean but some newer burners allow air to circulate around the glass and it keeps clean a lot longer.
My old stove has always been cleaned by a soft rag which is dampened and dipped in fine wood ash. It works a treat and doesn't scratch the glass if you make sure its always fine wood ash without bits in it.