At least 70% in most studies.Good Rules Of Prevention: Proper 'HANDS' practice
WASH your hands very well, often, with soap and water. Wash briskly and thoroughly.
While out and about, don't touch objects and then touch your eyes, lick your fingers, or stick them up your nose (for the children), etc...
Excellent tips - Also good idea to use antibacterial soap and water as hot as you can take it. In public, do stuff like punch elevator keys with your knuckle instead of the tip of your finger. And I keep a small travel-size bottle of Purell (or the like) in my car and my laptop bag when flying.
I believe it was said by the CDC that sanitizers had to be at least 60% alcohol to do any good.