lighter fluid, nail polish remover, alcohol, or 'goof off' after a hot water soak. Different glues will need one or the other .. and overall alcohol is the least effective. If the bottles are glass I use I plastic dish scrubber pad sometimes.
Yeah the dish scrubbers are a godsend for that crap, but only if the scrubber is molded plastic, and new. Silicone erasers (the pure white kind, artists use them because they don't contain silica) work well if it's not a whole lot of residue to remove. I really haven't had success with *any* chemicals, just plain elbow grease. If you have more money than time to spend removing that stuff, might be best just to trash the ones that are really stubborn, and buy new bottles. If you're more concerned with saving money than time, then it's definitely worth the effort.
I really only try to save the bottles that had Mylar labels, that way there's no paper residue left behind, just the adhesive -- if you can get the label off just as soon as you're done with the bottle's first use, it's helpful; the longer it stays on, the harder that adhesive is to remove.
Also -- if you have a 10ml bottle of, say, a flavor, that has a mylar label, you can actually re-use those labels on larger bottles -- I've done that with a label for INW Shisha Strawberry -- took the mylar label off the 10ml when it was done, and applied it to a 30ml bottle, which I use to pour some from the 100ml bottles of shisha strawberry I buy now -- that way the original 100ml bottle never has a syringe stuck into it, potentially adulterating the whole contents.
Andria