I use a shape usually, though some are also just a hint to the character themselves. any number ending in an 8 (08, 18, 28, etc) is a female character, as the 8 looks like an hourglass. In case of multiple female characters, it is the one that pays the most attention to her looks. In the marvel category I got the following:
40 - Iron Man (light on his chest looks like an 0
41 - Hawkeye (1 looks like an arrow)
42 - Angel (two wings)
43 - Wolverine (three claws on each hand)
44 - Beast (4 looks like a single nail of a claw)
45 - Thor (5 has slightly the shape of his hammer)
46 - Hulk (6 looks like an arm with a clenched fist)
47 - Spider Man (7 resembles a string attached to a ceiling, with which he swings)
48 - Black Widow (Probably the most well known female “superhero” from Marvel currently, because of the Avenger movies)
49 - Captain America (9 resembles a shield being thrown like a frisbee, like Cap does)
Numbers can also resemble the characters in a different way, 00 is the observer from Fringe, as the 00 looks like two eyes. 61 is Obi-wan Kenobi from star wars, as 60-69 is star wars good guys, and Obi-wan sounds like Obi-one. 99 is Leonardi da Vinci as 90-99 are historic people and Leonardo Da Vinci was a man with an incredible amound of talents, so he get the highest number. I could go on forever like that. 01 and 21 are Nina Sharp and “Red-Haired” Shanks respectively, as both only have one arm (although Nina has a robotic replacement, still one armed :P). 70 is Darth Baras as he is a fat Sith from Star Wars. 10 is Sherlock holmes for the same reason as the observer at 00.
The first number shows the category, and the second number resembles the character in some way.