Depending on what the description is for, it can either be a great help, or relatively no help at all. For example, if police require a description, they are usually looking for physical appearance traits, and anything "unusual" in physical features, mannerisms or characteristics. Clothing worn in last seen condition is important.
How to describe yourself might include:
Type of clothing last seen wearing
Additionally, describing one's self might include:
Unusual mannerisms (such as a stuttering, a face tick, won't make direct eye contact, slurred speach, etc.)
Unusual speech patterns, or perhaps an accent not familiar to the area, etc.
If the person is in the company of others, or a pet.
What is the mode of transportation of the person of focus: on foot, a bicycle, driving a car, on a plane, bus or train.
Condition of clothing or appearance: tattered clothing, dissheveled appearance, neat appearance, trendy, new clothing, etc.
While this approach would work for anyone describing another person it would probably serve as a good guideline for a self description as well.