I use a locally available commercial sign product called:
"Rapid Remover Adhesive Remover"
www.rapidtac.comIt softens old glue in about 30 seconds and you have work in stages. Use a plastic sign squeegee or ice scraper and lots of paper towels. Finish up with a warm, soapy water rinse & you're good to go.
It's water based & does not contain silicone, made in the U.S.A. and available from ND Graphic supplies on Annacis Island. Call them at 604-525-7940 Mon-Fri 8:30 - 5. Also available from Taca or any other local sign supplier.
Personally I would NOT use Goof Off, it's straight Xylene which is hazardous and will damage paint finishes. WD40 or other spray lubes may work depending on how old your decals are, they just take longer.
ALWAYS test an area away from the main part you ar stripping.
One big tip - try to peel off the decal rather than scraping them. Get as much decal adhesive off as you can before and you'll have a lot less to mess with.