Even when I do look at myself in the mirror, it's functional - eg I'm brushing my teeth - and how I look is about the last thing on my mind. I can not think of a single time I've stopped and addressed myself in the mirror with a detailed self portrait, except when it's the morning after the night before and I've caught sight of my blurry features and gone: I look so old, I'm never drinking again. Which, while half accurate (and you can decide which half), is hardly a helpful thing to convey to a reader how I look.
So, if staring at themselves with a running mental commentary on their looks is out for main characters, how can a writer convey to the reader how the character looks?
Get other characters to comment on your main character. This is from Kissing Mr Wrong:
Briony rang the number and after a bit someone answered. They exchanged pleasantries about the exhibition, then Briony said, 'I'm after a favour for a friend. Lu Edwards - I think you met her briefly at the party. Long hair, a bit hippyish.' Lu frowned at Briony. A bit hippyish? Just because she didn't wear black all the time like Briony.
Earlier it's mentioned that Lu is wearing a cheap skirt that she's customised herself with applique roses, which fits in with Briony's hippyish description. Or this from Adultery for Beginners:
She hesitated at the door, not daring to go in and meet the other parents. There wasn't a man in sight, she noticed, only mothers, and they all seemed to know each other. Some were dressed casually, others in suits as if for work. Isabel felt dressed too brightly, the colours bold and garish in the soft September light. Without thinking she touched her earrings, bought on one of their Dubai jaunts, bright Bedouin beads strung on gold wires that chinkled softly as she moved. She made a mental note to wear something beige next time.
The extract from Adultery for Beginners also uses comparisons to give an impression of how Isabel looks. If A thinks, I look thinner/fatter/happier/sleeker/untidier than X, and the reader knows how thin/fat/happy/sleek/untidy X is, they should be able to begin to imagine A. I think most of us compare ourselves to others in real life, so it seems entirely plausible that characters do it too.
Whatever method you use, it's best if main character description is drip fed into the story rather than ladling up a great wodge. I like to establish hair colour and length and basic body shape - tall, short, slim, plump etc - early on, along with a little characterisation through clothes. And that's about it. I think minimal description means the reader can project their own image onto the character and that in turn helps them get involved with my characters.
That's the theory anyway - and hopefully it works.