Is IT a strategic position ?
Are you part of Management Board, can you see and assess larger picture ?
Do you have a complete idea how business is going and what are priorities?
Are you able to see conflicting demands and you are confident about it ?
If answer to all above is 'Yes' then you are lucky and you can afford to say NO upfront.
But if IT is service department then it becomes tricky. Watch video from Gartner in below link as how to say NO.