Always take some time from the boss before making the decision
Never make hasty decisions under these kind of situations. If the request comes through an office mail, you naturally have some time to make up your mind and the consequent answer carefully, but when the boss asks directly, face to face or via telephone call, always take some time for consideration. This can help you out big time. Firstly, you superior will know immediately that you are having some doubts and secondly, it will give enough time to furnish a respectful no.
Respond according to the mood of your boss
In case your boss is not in a good mood when you are supposed to respond, avoidance can be the best policy. One way is that you can initiate some important discussion while other can be to make yourself scarce for a while.
Tell the boss the exact reason for declining
Saying no to the boss is one thing, but lying to do so is entirely another. Never make up a story in front of the boss. You should be very clear on that. Intimate your superior with the exact reasons and in case he was willing to give you some extra responsibility do not shy from paying him a compliment.
Give the boss an alternative solution
There might be some situations, when you saying no might affect the functioning of a workplace. While you are perfectly within your rights to make a selfish decision, a good professional never stops caring about the organization. In case do not deem yourself appropriate for a duty, it would harm to offer a backup plan. Also look at the possibility of a compromise and try to make a scenario in which both parties are happy.