Don't be afraid to look like a stupid human being. Don't be afraid to act it out, walking and speaking and making funny voices and sound effects. We all look quite funny when we play this, so there's nothing to lose, just to gain.
Yeah, I'm talking to you, you pal who always sits in the back and talks calmly. Why won't you act like your fellow player, who raises his arm like in pain whenever his being hurt? Why won't you act like Mellisa, who roses from her chair and shows how she's walking to the king and how she bows for him?
I'm talking to you also, you mighty GM. Why won't you give your players an example? You're the leading storyteller, the one that everyone looking to see what s/he will do, so please give your players an example. It can't hurt you, you know, it will only make you feel more energised.
I'm not telling any of you to go and play like this is a LARP (although it can't hurt to do it once in a while); I'm only asking you to try to live the game a little bit more. Getting up from the table once in a while, shouting and screaming like a barbarian when your barbarian is fighting, whispering something when you wizard does its magic, making a gun noises when your character's gun is shooting, but most of all: Talking like your PC and gesturing like your PC...