I know in Texas, if you get your windshield replaced, the place that does it has this spray that will loosen up the glue on the decal so that it can be removed safely and then placed onto the new one when it is installed.
If you live in a state that has windshield stickers.. registration or inspections stickers.. contact a company that does windsheild replacing and see if you can have them use their stuff to remove the decal for you.
I am sure someone else might have a way for you to do it on your own, but off the top of my head, that was the only thing I could thing of.
Jake