Take the sentence: I slid my long slender legs out from under the covers.
Another example: I twined my long, auburn hair into a practical bun.
And another: I bared my straight white teeth in the mirror to make sure there was no food stuck between them.
When you do things like this, it comes off sounding sappy at best. Sickly-sweet is what comes to my mind nearly every time.
Try to remember, you never give a lot of thought about your own appearance unless you are trying to make sure things are perfect in preparation for meeting someone important. A model probably will think about everything listed above, though the hair style would be something different.
That makes it kind of difficult for a writer to tell you what their character looks like. There are ways though.
It's easier for a girl, because a girl is more likely to be more critical of her appearance. You just have to decide if such personal criticizing is fitting with your character's personality. I can tell you from personal experience that a rancher's daughter probably isn't likely to care what she looks like much, not unless her mother is successful in girlifying her. My mother was far more successful with my older sister than she was with me.
There are men like this too, those who are incredibly self-conscious about about their appearance and those who are just lucky. My husband falls into the latter. I still don't look at clothes as anything more than a covering for modesty or warmth.
At any rate, how do you describe yourself without sounding shallow and selfish? There are all manner of subtle hints you can use, but basically it boils down to trusting your reader to develop a picture of your character that will do just fine.
Either gender might pick colors to accent eye color or at least not clash. A girl might do the same with makeup, maybe even selecting contacts to change her eye color to match her clothes. Hair color is another thing, but if your character is ever doing anything nefarious at night, they'd need to do something about light colored hair so it wouldn't show up. Having another character make a casual comment about hair color is another way.
The mirror thing works just fine, but you really have to be careful how you use it. Once again, you have to decide if your character is going to think about what he or she looks like or if they're just using the mirror for functional purposes.
Believe me, when it comes to self appearance, third person is best. I like third person close, it's very like first person, but it's outside of the body, rather like a gnome riding on your character's shoulder. Give it a try. You might like it. However, I do understand that some stories just hit better in first person.