One thing I do with any nut butter, is when I bring it home, I turn the jar upside down and leave it on the counter OUT of the fridge for a couple of days. That really makes mixing easier, especially when the jar's been on the store shelves for a while. Once you put it in the fridge, it's going to be really hard to mix.

You could always get one of those gadgets that has a mixing hook incorporated into a jar lid. But single-use tools always seem wasteful to me.