Don't get sloppy about protecting your food from nightime visitors. Bears can and will enter your camp to steal your food stash.
The easiest way to secure your food from black bears and grizzlies is to buy some large stuff sacks and place everything smelly inside. This includes not just your food but also your sun tan lotion, chapstick, dog food if you have any, and bug spray.
There are many ways to hoist up a bear bag. The one I prefer is to use two long ropes, throwing each one over branches of two trees about thirty feet apart. In the middle of the two trees I tie the end of each rope to the stuff sacks of food and smelling items. Then going from one tree to the other, I pull each line taut, raising my stuff sacks up into the air between the two trees.
The advantage of this method is that there is not a line going from the stuff sack down to the base of a tree, which smart bears can figure out, and just bite off. Instead the line goes to a a branch twenty or thirty feet up each tree. Also, the food is hanging in the space between two trees, out by itself, instead of hanging beneath a branch of a tree.
So far this method has been bearproof in the backcountry of Colorado, Wyoming, Montana, and Idaho.
I don't think it would work in Yosemite though. Those bears are so backpacker wise that the only method recommended is to smash your food into bearproof canistors.
I have heard some good stories about the bears in Yosemite. One of my favorites is how the bears have learned to wait until climbers ascending the base of Half Dome get up the cliff a ways, then come out of their trees and make off with the climber's gear sacks that have not been raised yet.
My experience is that the bears you have to worry about the most are the ones that visit areas where people camp frequently - like campgrounds or popular backcountry areas. They are attracted by careless campers, who leave things behind, and are sloppy with their food. You can go a long ways towards making your camp bear secure my avoidng these areas, and camping off by yourself.
Avoid typical bear travel corridors when setting up your camp - like the top of a ridge, the edge of a lake, the side of a stream. Out in the middle of a forest or in a wide meadow are good locations. Look around for bear sign before you set up camp. If you find bear scat, bear diggings, or claw marks on trees, it is a good idea to keep moving.
