There is no need to use harsh chemicals to remove labels and the glue residue they leave behind, making it more appealing to reuse or upcycle old jars. I picked up this neat trick from a blog posting at creekhouse.com and was very happy with the results.
1. Soak your jars in hot, soapy water.
2. Peel and/or scrape as much of the labels off as you can.
3. Make a paste of equal parts olive oil (or any kitchen oil) and baking soda.
4. Rub the paste onto jars and let sit for ten minutes.
5. Scrub the remaining glue and residue with the paste. On some jars, removing the residue is a little tougher. On those jars, use a kitchen scrubber.