Frog has mentioned it a few times, but a "warm" surface for weeding will help release the vinyl from the backing. I like the red picks from siser. I know they are a little pricey, but they are comfortable (to me).
I also started adding relief cuts to art, so the whole image isnt part of the same weeding piece. You can pull the negative area off of individual elements of a design in whatever way seems efficient, and it saves some time and hassle.