It may sound sound obvious, but the most important thing is that you like how a tie looks
Buy the tie that appeals to you, the tie that catches your eye. Which is how we all choose ties, anyway.
However, your tie souled not strike your fancy, but should also be appropriate for the occasion at which you will be wearing it. You may take a shine to that hula-girls and palm-trees number at the store, but unless you work in a Hawaiian souvenir shop, don't buy it!
Your tie should also coordinate well with your shirts, suits and jackets. Therefore, when you shop for a tie, do not evaluate it by the way it displayed, with other ties on the rack. Consider how the tie's colors, design, fabric and width, would complement a specific shirt or suit.
Try to wear the clothes you have in mind to pair it with. But, memory can deceive. The most striking tie may not match any thing in your closet. If you are not wearing a formal shirt or a jacket when you are buying a tie, compare that tie to those in the store that work well together.