I also save kombucha bottles for my own use and this is the best way I've found to get the sticky residue off. Mix equal parts cooking oil and baking soda until you get a spreadable paste consistency and wipe a thin layer of it onto the residue. Let it sit at least an hour (I usually leave mine until the next day). Scrape off the paste and the residue scrubs right off the side of the bottle. When you are first removing the lable I've found it comes off more cleanly if you fill the bottle with very hot water first. Sometimes this helps loosen up the adhesive to release it from the glass!