If you've rented a wallpaper steamer to remove the old wallpaper, fire it up and blast it full force on the remaining wallpaper glue. Have a flat-edged scraper handy in your other hand, and follow the steam with some scraping as you go. The key is to catch the glue when it is at its softest. Immediately after blasting it with the steamer, it will be very soft and easy to scrape away from the wall.
Purchase some tri-sodium phosphate (TSP) at your local hardware or home-and-garden store and mix it per the manufacturer's directions with some warm water. Apply the solution with a rag or sponge and soak the area where the wallpaper glue remains. Once it has softened, scrape away any residue with your scraper. Rinse a rag in the solution and clean the wall one final time after the glue has been removed.
Goo Gone is a citrus-based product (one of several available at mass retail stores and home and garden centers) that will work as a solvent for many types of glue. Apply the gel thickly to the wallpaper glue and/or residue, and wipe it away with a damp sponge. You may need to repeat the process a couple of times, and scrape the residue with your scraper.