I'm sure there's a much more elegant way to go about it, but I thought I'd share what I do, sometimes with friends to make pc group dynamics interesting. Or at least my attempt. I mean, rules can only do so much.
So, I have everyone write down an aspect, like a trait. You guys know what I mean. I give 'em a sharpie and a somewhat small index card. Like, those half-sized ones. This way they can't write too much. I don't want a sentence or even three words, but one word or maybe two is just fine. I think one word is best, but sometimes people have trouble distilling something down to a word.
So after they've written their important trait down, I have them show everyone what they wrote. This way, they have a thing that is their own.
Then, I ask everyone to make opposite words for each trait. Sometimes a trait will have a couple different approaches. Sometimes there's no clear cut opposite, but a bunch. That's okay.
I ask everyone to choose an opposite trait. I ask that they please not choose the same one that someone else has chosen. If that means, taking the little piece of paper that the trait is written on, that's okay. If you can work this out without much fuss, that's great too.
The third step is someone writes a trait for your character. You have to accept this new trait. You can decide this however you like, but getting everyone involved is I believe the best thing.
It coaxes people into letting go of creative control. Also, it get's another player invested, and it might give a person the sense that someone is interested in their just-made-up thing.
Someone is interested in your idea.