In general you want to look at what your job description is. If this goes outside the job description then you have a argument to why you shouldn't be required to do it. In all of my experience you're not going to be better off trying to go this route unless it's a big company and a higher level position.
If you're looking for more pay for the extra job responsibilities then be a diplomat. Set up a formal meeting with your boss. Don't make it informal or they will not take you as seriously. Know what you want, know what you'll settle for. Make sure you directly ask for more compensation rather than hinting. Explain that you're excited about the new responsibilities but you'd also like your compensation to reflect your extra efforts.
I've made mistakes with the way I did the meeting (I did an informal meeting and came unprepared) and lost out on extra compensation before.