I know that when I create my sim, whether it is the actual body, traits or clothes, it is based on what type of sim I am planning. Is she going to be a business type person, an outdoor type, explorer?
I wouldn't take a sim and give her long loose hair, dress her in a fancy dress, high heels and send her into the tombs to hunt for treasure. She will be wearing jeans, good hiking shoes, casual shirt, hair cut short or in a braid to make it more manageable. Is she going to be outside, then tanned skin. A career that has her inside, lighter skin color. Is she supposed to be from a specific type of nationality? Then I try to make her or him look as much realistic as I can of their specific appearance. Each race has a special feature that says, hey, I am of this race, shape of eyes, noses, mouths, hair type, think stereotype of what a person from any country is seen as looking like.
Your athlete, is the sim a basketball player, football or other? What type of physical makeup is needed? Football players, for the most part are more muscular whereas basketball are more lean, muscles not as defined.
And as stated, think of the career and match traits, a military or law enforcement won't want someone who is a slob and couch potato, they want brave, disciplined, athletic. An author doesn't need to be a party animal, but computer whiz and bookworm. Toss in something for fun, insane, for example.
Think ahead of what you may have in the future also, will you be having a Bonehilda? A neurotic sim I had was always running screaming from her.
Hope this helps