Stay out of areas that are closed for bear management.
See the bear before you surprise it. Watch for fresh tracks, scat, and feeding sites (signs of digging, rolled rocks, torn up logs, ripped open ant hills).
Don't hike alone
Hike in groups of three or more people. 91% of the people injured by bears in Yellowstone since 1970 were hiking alone or with only one hiking partner. Only 9% of the people injured by bears were in groups of three or more people.
Don't hike at dawn, dusk, or at night
Avoid hiking at dawn, dusk, or at night: during summer, that's when grizzly bears are most active.
When hiking, periodically yell "Hey bear!" to alert bears to your presence, especially when walking through dense vegetation/blind spots, traveling upwind, near loud streams, or on windy days. Avoid thick brush whenever possible.
Carry bear spray and know how to use it
Bear spray is proven to be highly successful at stopping aggressive behavior in bears.
Don't expect bears to see you first
In Yellowstone, bears hibernate for approximately five months each year and have only seven months of active time to obtain all of their nutritional needs. A bear that's feeding may not see you as quickly as you would think. Pay attention, and see the bear before it sees you...and before you surprise it.
Stay on maintained trails
Research in Yellowstone has shown that people are more likely to be attacked by a bear when hiking off-trail.
Bears will guard and defend carcasses against other scavengers or humans. Dead ungulates will attract and hold many bears near the carcass site. It is risky to approach a carcass; many bears may be bedded nearby just out of sight. If you find a fresh carcass, leave the area immediately by the same route you approached. Report all carcasses to the nearest ranger station or visitor center.
Stay with your stuff
Do not leave packs or bags containing food unattended, even for a few minutes. Bears learn new food sources quickly. Allowing bears to obtain human food even once often leads to them becoming aggressive toward people when they come back looking for more. Aggressive bears threaten human safety and eventually must be removed from the park or killed.