Dark wash jeans, solid colour t-shirt, a collared shirt with a subtle pattern buttoned up over top of it, and depending on the temperature, no jacket, a corduroy blazer, or a pea coat. If it's really cold I'll put on a wool touque and a wool scarf, and if it's really hot I'll switch out the collared shirt and t-shirt for a polo shirt.
My basic rule is that I like to be one step better dressed than the rest of the room. If most people are wearing t-shirts and hoodies, I'll put on the collared shirt. If most people are wearing shirts, I'll grab a tie.