As a long time GM I've come up with some standard house rules designed to award experience (Karma, character points, pick your flavor) to individuals who go above and beyond to get into the roleplaying of their character.
For our D20 games if a player brings a prop, any prop, they get to add 10% to thier awarded experience points so long its its brought to every gaming session. You might think that some players would cut out a cheesy paper symbol to tape to their chest as a prop, and I have one fella who does do this, but I've found that more folks go to some lengths to find good props to bring. One of my players is a Master Pipefitter, and he forged a Mace to represent his Holy Mace of Lathander using pipe, fittings, and welding tools! Another player sewed an incredible set of blue satin robes for his Cleric to represent his order, and created a beautiful replica of his holy symbol in wood and painted it gold. Complex or simple, the props really make the gaming experience better by helping the players get into roleplaying their characters.
Another thing I do is award "Consumable XPs" to any player who brings a dish of goodies to share with the entire game group. This eventually turned into what we call "Stocking the Treasure Chest", an actual chest I keep in the game room that is bursting full of candies, chips, cookies, and bottled waters to be consumed at game sessions. Those who contriubute earn XP that is put into a group pool. This xp, by group consensus is used to tip players whose characters are close to levelling over the edge, make magic items, or whatever else the party may need extra xp's for. This way everyone gets to partake of a large selection of goodies each game, and no one is in the frustrating position of being 27 xps away from levelling for a game session.
