What's your experience level? If you know what you're doing, it's worth paying a little extra for a high-quality model that will perform better and last longer. If you're a novice, you'll probably be more comfortable with a mid-range model that's less expensive.
Which accessories do you need? The size of your blades and comb guides are critical to getting your hair cut to the proper length. You can almost always purchase additional combs or blades, so focus first on getting quality clippers. If you're choosing between two similar models, opt for the one that has the length of combs you need.
Who's the clipper for? If you need a clipper to groom dogs or other animals, opt for a purpose-built model. Pet hair -- especially dog hair -- is usually a lot thicker than human hair, so you'll need a more powerful model to cut through it comfortably and efficiently.
Who's going to cut your hair? If someone else is cutting your hair, almost any models are fair game. If you intend to cut it yourself, however, stick to the lighter models or a clipper with a swiveling cutting head.
Are your mirrors properly set up for hair cutting? If you're cutting your own hair, a mounted swing-arm mirror on a wall perpendicular to your stationary mirror will allow you to see the back and sides of your own head without constantly holding up a mirror.
Experts say that brushing off hair after every use is critical to the life of your machine. Oiling the blades after every other use will also keep them working properly. Most models have self-sharpening blades that never have to be replaced.