One technique that is commonly used in fiction of all sorts is voice. A character with a strong voice, or distinctive vocal mannerisms and tendencies, sets the character apart from the rest and makes it simple and easy to pick the character out from a crowd. In comics, strong voice is especially useful and important. There are lots of great examples from comics, but the voice I'll use as an example is Mace Blitzkrieg from Comics Greatest World/Dark Horse Heroes:
We got unfinished business, cape monkey! You think you can come to my table uninvited and eat off my plate! Wolf Gang is my dessert. And Mace Blitzkrieg don't believe in sharing!
This is extremely strong voice. From this one line, I can imagine exactly what Mace Blitzkrieg sounds like-a hoarse, constantly angry, constantly shouting man. He asks a question, and it's still punctuated by an exclamation point, suggesting exactly how the line would be delivered. And even though I have only one comic with Mace Blitzkrieg in it (Will to Power #6), I would be able to identify him in another comic in a heartbeat.
If you're wondering why I wouldn't use a more famous comic example, such as Doctor Doom, it's partly because he's a much more famous example. (There's also been the hubbub lately over Doom's line to Ms. Marvel which most view as out of character; I'll just say that while Doom tends to insult folks, he usually stays civil-and that wasn't civil. Of course, if I understand correctly, he was under a lot of stress.)
As a further note, imagine if a character had the following line.
My life sucks. Suckity suckity suck. Suuuuucks.
While this might be an appropriate line for a character with a life as hard (comparitively speaking) as Spider-Man's, if Peter Parker actually said this, there would be an uproar. Why? Mostly because it doesn't fit his voice. (Okay, if he's ever actually said that, apologies for being wrong.)