Check for symmetry. Adjust the knot so that you achieve optimal bilateral symmetry. Re-tie or upgrade to the Windsor if necessary. A slightly leaning knot can be wriggled with so it looks OK, but a seriously cockeyed knot will have the tie falling down your lapel and just look terrible. There are exceptions to this rule. Some knots are meant to be asymmetrical (often worn in Italy, for example).
Keep it tight. A properly tied tie will, unfortunately for the wearer, need to be pulled extremely tight against the neck. A tie too loose may convey unprofessionalism.
The Vicious V. According to the film, Outside Providence, a properly tied tie must possess the Vicious V. This is the sharp V formed by the scruching of the tie where it exits the knot.
For the tall. Tall people will find that the knot sometimes will end up way "up" on the tie where it is narrow resulting in a small unflattering knot. You may want to look for specially made ties for tall people that will solve this problem.
Be sure to dimple. Where the wider end of the tie exits the bottom of the knot there should be a clean symmetrical dimple.
Untie your tie. Untie your tie after you are through wearing it, also untie it the same way you tied it. Your tie will last longer.