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. You can always upgrade when you're more comfortable cutting hair yourself.
Which accessories do you need? The size of your blades and comb guides is 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're cutting small children's hair, you may want to opt for a smaller model. Parents also like the versatility of cordless clippers so they can take their barbering outside and keep the mess out of the house. If you're cutting an adult's thick or curly hair, purchase the most powerful clipper you can.
Will you be cutting your pet's hair? Plenty of people buy hair clippers for the sole purpose of cutting their pet's hair. If that's the case, focus on clippers that get the best reviews for being quiet and vibration free, so you don't freak out Fido or panic the puss.
Who's going to cut your hair? If someone else is cutting your hair, almost any model is fair game. If you intend to cut it yourself, however, stick to the lighter clippers or one with a swiveling cutting head, which makes it easier to reach the sides and back of your own 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.