I've bought a few commercial bear bags in my time and found them lacking in various ways. So I assembled the following bear bag system that I've been using very successfully over the past 2 years. It has 5 components: a ultralight stuff sack capable of holding 4-5 days of food, 40 feet of reflective guy line, a mesh throw sack, a mini-biner, and an OPSACK. All in, it weighs 4.0 oz, just 1 oz more than Backpackinglight's Ursalite system, but it is higher capacity and far more sturdy.
The exterior bag is a XL Spinntex EXP Stuff Sack manufactured by Mountain Laurel Designs that weighs 0.5 oz and holds up to 4 days of food. It is made using the lightest and strongest fully waterproof conventional woven spinnaker fabric available and can withstand hard rain. It has a separate hang loop and cinch cord. Not having to use the cinch cord as the hang loop extends the life of the bag and the hang loop is bar tacked to a vertical seam for extra strength. These stuff sacks are a little noisy when new they but get less crinkly and quiet as you use them.
Next up, is the throw bag that you put a rock into and throw over a tree branch. (When I can't find a good size rock, I often use a can of tuna fish.) I've found that a small mesh sack lasts far longer than silnylon throw bags which shred on impact in rocky New England. A small mesh sack is also far easier to repair with small bits of string.
The next step is to tie the cord that your bear bag will hang from to the throw bag. There are lighter cords that you can use for this, but I like Kelty Triptease Lightline because it is easy to unknot and it has embedded reflective elements in it that will catch your flashlight at night. It's easy to forget exactly where your bag is hanging in the woods and the reflectors make it much easier to find if you need to access it after dark.
When you tie the cord to the throw bag, add an extra loop for a mini wire-gate carbiner, as shown above, that will clip to the hang loop of the bear bag. These mini-biners just weigh a few grams and are perfect for this application. You can get them from backpackinglight.com and lots of other places. I also use them for hanging little things off my backpack that I use frequently like a swiss army knife, thermometer, and emergency LED.
In the absence of a tree, here's what the biner, cord, and hang loop look like in full deployment. After you've found the perfect tree branch and thrown the line over it, attach the biner to the stuff sack's hang loop as shown. Pull the bag 10-15 feet off the ground and secure the other end of the line to a different tree above head height.
What about the OPSACK? I use this to line the inside of my bear bag and then put all of my food in it. It is 17,000 times less breathable than a normal ziploc and reduces the chance that a bear can smell your food. Bears have a very good sense of smell.
There you have it. What do you use for your bear bag system?