What @bilvyy said.
Eyebrows play a huge part in making an expression feeling real.
Also, smirking.
I think you're gotten a great start already!
Having the eyes look more or less dead (flat on the up side) helps the facial expression to look a bit more sinister, which is a part of being a "smart ass".
Having the head point downwards also helps, making the character look more secure in themselves. Pointing upwards usually mean that they think they're above everyone else. It's just my opinion tho!