In fiction, the main character is often the first person we meet in the story. Given that the story is written from his/her point of view, how would you describe what they look like? The classical tricks (they look in a mirror or see their reflection in a lake) are not allowed (too traditional and boring). Do you have any other tips or tricks?
Give the character a good reason to think about or comment on one key aspect of their own appearance. Perhaps the MC is proud of achieving something related to looks. Or jealous of another character. Or concerned about what some specific person might think. Maybe something has changed, by choice or otherwise. Or perhaps the character has to consider some physical attribute in order to make some decision. The important thing is to make it a strong reason that relates to the story.
Give another character a reason to comment. Maybe something has changed. Maybe the other character is envious. Or something.
Give the character some action that demonstrates or suggests physical attributes. Do they have to move the car seat back in order to fit behind the wheel? Or move it up to reach the pedals?
Leave something for the reader to fill in. Maybe leave all of it for the reader to fill in. I saw Michael Connolly speak once, and he said that there was some main character (maybe Harry Bosch, maybe some other) that he had never described.
One important point: If you are going to describe some distinctive feature, do it before the reader has filled in their own image. Learning on page 112 that the character is 6'5" tall can be jarring if the reader has imagined the character at average height.
On one side of the spectrum, some ways of describing have the particularity that, instead of describing all of the character, they define them little by little.
I. You can highlight their body while they do something.
(Implying a tall character)
(So clearly it's a muscular character)
II. In the middle of a dialog, as a description:
III. Have someone make remarks about the characters.
Of course, you can mix those. For instance, in that last example, you could add "I felt my face was getting as red as my hair" or something similar.
On the other side of the spectrum, you could use a technique that would be applied differently depending on the psychology of the character. For instance, you could have a very self conscious teenager reflect on their appearance; this approach would apply for a trendy character, too, although with a different feel (critically checking whether they look as they think they should, for instance). As far as I can tell, there are a few more character types that can have this technique easily applied, such as a narcissistic character, or simply one with high self esteem.
The "main thing" about this second approach is, depending on the characters' self esteem and general attitude towards life, and whatever number of factors, the general feel of the way they think about their appearance changes.
This isn't really any different than any other important information you want to get across early. Here's a few thoughts:
Hope this is helpful :)
All of the previous answers are good.
One technique I didn't see mentioned is that the reader can infer some of the POV character's looks when the POV character compares him or herself to other people, such as:
"Wow, that guy has even curlier hair than I do!"
"That guy must be loaded. I've been shopping big and tall shops since I was 16, and I know suits like that don't come cheap."
"Her mouth looks like mine before I got braces."
I subscribe to the "John Steinbeck school-of-thought" on character description, as set forth in his prologue to Sweet Tuesday:
Consequently, when I describe my characters I try to only include descriptions that are