I usually just pick up one corner and pull very slowly, making sure that if the tag crosses the edge of the label, I'm not peeling in a direction that risks lifting/delaminating the edge of the label (e.g. I usually peel from teh center of the label outward rather than inward against the edge of the label). If the force is slow and even, I've usually found I can remove stickers from other stickers and glossy paper surfaces without damage, if the underlying surface is in good shape.
I've also seen the team at my local vintage resale shop take a heat gun gently to stickers to affect the adhesive before trying to remove them.