In most parts of the country correctly hanging food in a bear bags can be effective though it takes a bit of skill. Unfortunately bags won’t protect your food in the more highly trafficed parts of the Sierras were the bears see backpackers and think “Great, I get another treat filled piñata tonight”. In the Sierras protect your food and the bears by storing your food in bear boxes or using a bear resistant container such as the BearVault (nice and reasonably priced), Wild Ideas Bearikade (lightest hard wall, and expensive), or the Garcia Backpackers’ Cache (the original which I am not fond of). Bareboxer makes some smaller canisters which are good for short trips. Andrew did a nice analysis of bear canisters volume vs price and price.
Photon sent a nicely written email to backpackinglight mailing list suggesting that you should use canisters in the Sierras. Andrew Skurka more recently wrote an argument against hanging bear bag. Why use a can? First, it might save the life of a bear. Those which get used to raid people’s food will eventually be destroyed. Second, loosing your food, especially 4 days into a 9 day trip really sucks.
There are some people who advocate “stealth camping” which is stay away from camp sites that bears habitually visit, cooking your food before you get to your campsite, and sleeping with your food on the theory that a bear will be less likely to bother a human and you will be in a position to defend your food. I don’t recommend this in areas with black bears, and I think anyone is insane to do this in brown bear territory. For extra insurance some stealth campers make sure that smelly things are stored in something like O.P. SAK Barrier bag which look a lot like a normal zip-lock bag, but is in theory, odor-proof.
Another option Ursack which is basically a stuff sack made from very strong threads. The ursack has been on the “conditionally approved” list periodically, but hasn’t lasted more than a season or two before the get pulled. So I wouldn’t recommend them in places with rigid rules… but they maybe be appropriate and effective in more lay-back areas. Caveat: Ursacks are best when you don’t encounter a bear. If a bear finds the sack it will try to get at your food which will get crushed. This hasn’t happened to me, but I have talked to several people who ended their trip early because their food was a mess after a bear tried to access their Ursack.
A few years ago there were some portable electrified containers such as the Palisade EST that didn’t make it in the marketplace.
There isn’t a single organization that is managing bear canister policies in the Sierras anymore, but the is a list of the places that have bear canister requirements in the Sierras. You can find useful information at the Interagency Grizzly Bear Committee. Eastern Slope Grizzly Bear Group and USGS Alaska Science Center Bear Project has good information about grizzly safety. There is a nice map of where sierra bear boxes are located.