I'm going to refuse to cover my co-worker's shift for an important event.
Is there something about your co-worker that you don't like?
I think he is a lazy bastard. He never helps me with my shifts.
While it is understandable to not want to help out a co-worker when you feel slighted that you haven't been helped in the past, it is still important to be a good employee and ensure that your workplace operates smoothly for the sake of the business. You should raise your concerns with your co-worker and how you feel that you have been helping him out much more than he has helped you. Ensuring that there's an equitable division of labor going on is important for a healthy working relationship.
No matter what you say, I will not help him out.
I really think you should reconsider helping him out. I understand that you may not think the highest of him, but if you help him out, he would be obligated to do the same. Talk to him about how you want to help, but would like to be helped in return.