How do you use it? Are you taking it out to a job site each day? Or, is it for general use around the house? The first choice requires a heavy duty unit. If you are taking it to a job site will it be heavy drilling? If so, then make sure that you purchase a nineteen volt model with a keyless chuck that will take up to a half-inch bit.
What features do you need? Most units will also double as a Cordless Screwdriver which can save some money. On the other end there are some that are compact enough to work in very tight spaces. They average four to nine volts and are cordless drills in name only.
If you are looking for long battery life, then Lithium Ion batteries are the way to go. Also, you want to check to see how much the battery costs. The Sears EX 12volt cordless drill was only forty dollars. The battery was twenty. When the EX was discontinued, they also discontinued the battery. When the one battery that came with the cordless drill went out you were stuck with a paperweight.
How much do you want to spend? A Cordless Drill can range in price from under ten dollars to over two hundred. Generally speaking, a high end Cordless Drill will have variable speeds, a more robust transmission, a lithium ion battery with a quick charge feature, and a keyless chuck. Look at a Makita Cordless Drill for examples of what a good quality mid range cordless drill should cost. I personally own and use one every day and it will drill, drive screws, run down bolts, and the battery lasts all day.
Also, more cordless drills are starting to come with a built in work light as well. Until you use it, you won't realize how handy this feature is. Make sure the light is made up of LED's instead of a traditional bulb.