Measure your laptop. You may think that a 15-inch laptop needs a 15-inch case, but not all 15-inch cases are created equal. Manufacturers and retailers vary in the way they describe sizes, so a 15-inch widescreen, for instance, may not fit a bag that holds a regular 15-inch laptop. Check the actual measurements of your laptop and compare them to the laptop compartment -- or better yet, bring your laptop to see how snugly it fits.
Where do you take your laptop? If you work in the corporate world, you may need a high-style case that fits in with a business wardrobe. By contrast, if you'll be using the bag outdoors a lot, you'll be better off with something sturdy, utilitarian and waterproof. If you plan to use the case for long-distance travel, consider one with quick-removal options that can speed your passage through airport security. You may also want a case that's large enough to hold a few personal items, like a sweater or a sandwich, along with your laptop -- but not too large to meet the airline requirements for carry-on baggage.
Balance weight with durability. Sturdier laptops can be heavier as well, so consider which trait is more important to you. If you're just looking for something to tote your computer back and forth to work or school, you probably don't need as much protection as you will if you're planning to take it out hiking and write a blog entry from a mountaintop. If you need a bag that's both tough and lightweight -- for commuting by bicycle, for instance -- then be prepared to pay a premium for it.
How long will you keep this laptop? In general, the more you pay for a laptop case, the better made it will be. However, that investment could be money down the drain if it's only going to be a year or two before you upgrade to a new laptop that may not fit your current case. This is especially true if you choose a bag with a custom-fit laptop compartment. This feature will protect your notebook from bumps and from shifting around as you walk, but it could make the bag obsolete when you upgrade. If you change laptop computers frequently, consider using a sleeve inside a larger case, so that when you swap models you can just replace the sleeve instead of the more expensive outer layer.
The very best laptop cases, typically made from leather or thick ballistic nylon, can cost hundreds of dollars. Bags of this quality are often backed by limited lifetime warranties (though these will only cover manufacturing defects, not abuse from users). Cheaper cases may still have decent warranties, but it's unlikely you'll find it worth the trouble of digging up a receipt and returning them to the maker. If the warranty on a bag is a big consideration for you, take the time to look at what users have to say about the company's customer service, since experiences vary widely.
Shop comparatively. The same case may go for one price on one website and half that on another. We frequently find sales that don't extend across all sites.
Count up extras. Some cases start out less expensive, but rapidly go up in price when you add on accessories (shoulder strap, strap pad, interior sleeves) you want. Others include those features in the base price. Be sure you're seeing the true bottom line when you compare prices.
Don't forget manufacturers' sites. On some company sites, you can score free shipping or extra accessories. Color and style choices may be more extensive. High-end brands such as Tom Bihn may be sold only through the manufacturer's site, where they can be customized.
Consider skipping the sleeve. Some high-end cases include a sleeve, but will reduce the price by $20 to $30 if you opt to skip it. If you already have a sleeve or don't need one, this can reduce the cost of a high-end laptop case.
Order accessories when you buy the case. Typically, it's less expensive to buy any accessories you need at the same time you order the laptop case. You'll likely get them shipped free as part of the package, and there may be other discounts as well.
Get tips from bag makers. About.com's Melanie Pinola asked notebook-case manufacturers what advice they had for shoppers. You can see the resulting FAQ on About's Mobile Office site.
One of the biggest frustrations of carrying a smartphone or tablet is running out of power when there's no place to plug in and recharge. Now, a new breed of laptop case offers a solution: a rechargeable external battery built right into the case, so you can store your devices and recharge them at the same time. The ones we've seen typically pack enough juice for two full charges or multiple partial charges on the average smartphone. One brand, called BirkSun, even includes solar panels to power up your devices directly from the sun. So far, however, we haven't seen a charging case that can recharge the laptop itself.