Our road tax sticker is pretty sticky, with a layer of adhesive that glue it firmly onto the windscreen. When it is time to renew the road tax, many find it difficult to peel off the existing sticker without leaving stain of glue and/or small pieces of the sticker.
I found that WD-40 is pretty useful in helping us to peel off the existing road tax sticker from the windscreen cleanly, without leaving any stain or residue. Just gently peel off the sticker as usual, and don't worry about the adhesive and residue that is hard to remove. Then, spray some WD-40 onto a piece of dry cloth, and wipe them off from the windscreen. Now, you will have a clean space to put on your new road tax sticker.
Besides, I have V-Kool tinted on my windscreen, and I found that this WD-40 (sprayed on cloth) will not harm the tinted film, and hence this method is safe to use on tinted windscreen too.