The most common problem managers speak of is too much to do and not enough time to do it in, and one of the things that really compounds this problem is the inability to say “no” effectively.
One simple tip is to assert your right to ask for clarification or more information. This will buy you time, and allow you to weigh your options and your priorities. It will also make it clear to the other person that you’re not agreeing to their request straight away.
My best wishes, Paul