Honestly, I think the very best way to do what you're looking for is to select one primary personality trait (preferably negative), and do your best to play through that prism all the time. Even to your own detriment as a character. All your focus goes into playing that trait as strongly as you possibly can.
The game 7th Sea had this. All characters had either a positive arcana (called a Virtue), or a negative arcana (called a Hubris.) And players used that one arcana to color everything about their personality. It seems easy, but in truth, roleplaying a character where there is this overwhelming flaw (that you yourself do not have) that influences all your actions and interaction is rather difficult. You always have the urge to ignore the Hubris when it might cause a complication for you or the group... but when in truth your character wouldn't be able to ignore it because that's just the way they are.
Hubrises included:
Ambitious: You chase after power, even when its risky
Arrogant: You show contempt or disdain for someone else.
Cowardly: You avoid or escape from doing something dangerous.
Envious: You covet something belonging to someone else and should attempt to get it.
Greedy: You try to get as much money as possible when divvying up loot, getting a bribe, etc.
Hedonistic: You relax your guard and have a good time.
Hot-Headed: You fly off the handle and lose your temper.
Inattentive: You do not pay attention to your surroundings and lose focus.
Indecisive: You can't help but take your time in making decisions or taking action.
Judgmental: You jump to conclusions and form unsubstantiated opinions of someone you've just met.
Lecherous: You give in to temptation.
Loyal: You go back for a fallen comrade, or avoid leaving their side in the firstp lace.
Misfortunate: You always seem to have bad luck follow you around.
Overconfident: You don't have any doubts about your capabilities.
Overzealous: You strongly defend one of your opinions, no matter how inappropriate the time or place.
Proud: You refuse offers of aid.
Rash: You investigate something unusual, even if it looks dangerous.
Reckless: You ignore any feelings of worry or hesitation when confronted with danger.
Righteous: You don't have any doubts about the moral justifiability of your actions.
Star-crossed: You are shot with Cupid's arrow... again.
Stubborn: You are prevented from changing your mind.
Trusting: You lose any doubts about another person.
I think oftentimes when we create characters, we think up long lists of character traits, both positive and negative. "I'm this, and I'm this, and I'm this, and I'm this..." but what happens is that you end up not playing all your traits, or only play them part way and never that strongly. But when you focus on just one... and everything you do is in reference to it... you find ways to behave you never reached before, because you have to continually dig deeper to find ways it influences you.
That's my suggestion at any rate.
