NO is not the BEST word as the 1st word of your answer.
It conveys a NEGATIVITY instantly to your
colleagues, your boss, your family members.
If "NO" is the only viable solution , say "NO".
However, don't start with "NO,....."
Remember.
You can DELIVER a "NO" while actively adopting
a POSITIVE approach to solving a problem.
If "NO" is the best answer, don't say "Yes".
However, first and most important , convey
your sincerity ,
your determination ,
your alignment to the common goal
your resourcefulness to solve the problem.
Then spread the logic , your basis of saying "NO". This approach brings forth understanding of your position by your counterpart. In situations, this may even lead to a solution that is Win-Win for both. Even if you don't come to an agreement, it is known why you said "NO".
Be sensitive to use of your language.
It does not pay at all being disrespectful even when you strongly disagree on a view point.
Be resourceful.
Offer a solution with sufficient background and support material , that is better than the current proposal.
Do, praise the good points of the opponent's proposal while highlighting better aspects of your proposal.
Be ready to negotiate hard.
You can't win a battle by turning your face away from the battlefield. You must hold the ground patiently to put forward your view.Be persistent.
Show humility
However convey STRENGTH in your voice and body language.
Self Check : Do you start like this ?
"No, I would like to say...."
"No, It is impossible to do it in such short time..."
"No, no, no, you can't do it this way ...."
Observe carefully , better ask feedback from reliable friends, colleagues and family members.
Strange it may sound, some person habitually starts with "NO" , even when they mean "YES".
If you happen to share this habit,
throw it like a rotten apple in a compost-bin.
Wear a new face of positivity and YES apple.