How to refuse things.

There are ways to say no politely. 

There is no need answer

immediately.                             

Take some time to reflect and decide if you can or want to please the request and then give your answer. 

Another technique is to suggest an honest alternative that makes sense and reflects your desire to help.

Do not forget that if you have agreed to do something and you can not keep, much worse for everyone than if I had said no to the begin. 

Also remember that it is more difficult to change a "yes" for "no" than to change a "no" for "yes" and do not forget that you can say no with a smile.