How do you politley refuse a tip, especially if they insist you take it?

I ask this and my previous question because I am a driver for Fedex, we don't expect tips, this is our job afterall to get your packages to you as quickly as possible(hopefully) I feel awkward when people try to give me money, so how do I refuse that without being rude? In my opinion if you want to do something nice for your driver I'd say just be nice in return and please understand if we are sometimes late, most of the time it isn't your drivers fault. But don't put up with bad service either if your courier is rude to you or does something that you think is unaceptable then call in and report them. I take pride in doing an awesome job for my customers and if they were unhappy with me and didn't want to tell me to my face I would want to know what the problem was so I could fix it. Oh and if you feel it nessacary to give a gift, I like candy,and pens, we always need pens lol, but please no money