For both my dogs I taught them by first saying, "Shake a Paw" and then I would take their paw and shake it. Immediately I praised like crazy and gave a treat. I repeated this again, waiting a second for them to give me their paw, if they didn't I would take their paw, shake it, and again praise like crazy and reward. It only took a few times before they were both giving me their paw after I said the words and held out my open hand.
Jazz, Shiloh Shepherd, 5 yrs old, CGC HIC
Bunny, GSD X, 7.5 years old