1. Stains and Rips
If their shirt is stained, you better believe girls are going to notice, and if the shirt doesn't get washed they'll remember. The same thing applies to rips. Take better care of your clothes, boys.
2. How they smell
If they're wearing far too much cologne, their conversation with a girl is going to be short. Also, if it's obvious they are wearing cologne to cover up a bad smell underneath, that's definitely a deal breaker. Smell good, not overwhelming.
3. Where they look when talking to you
If their eyes are all over the place, a girl is definitely going to notice. Especially if they're staring off into the distance. Pay attention.
4. Posture/How they sit
Plopping into a chair leaves a bad impression, as does excessive slouching. Is it that hard to straighten up so you don't look like the world's foremost slacker?
Nails that are dirty leave a bad impression. It's obvious if a boy bites their nails, and that's very off-putting as well.
6. How they walk
You trying to show off your swag? It ain't working, bro. Just walk like a normal human being. It works.
7. Who they hang out with
This is the big one. If all their friends are jerks, girls take note. And it all their friends are girls, that may be a clue off that they're a flirt, depending on who the girls are.
Don't think girls don't notice, because they do. Just be conscientious. You'll be fine.