How can I get my Pomeranian to stop nipping me?

I just adopted a 10 month old Pomeranian from the shelter. He's really sweet but I'm not sure if I've established enough authority over him. He sits when I command him to but doesn't always jump off the couch when I say no. Sometimes when I'm working I need to be alone and he won't go away. I give him lots of attention during the day but when I try to push him away he nips me with his teeth. I really don't want to give him back to the shelter. He listens to my boyfriend but not me. HELP!