You should provide a valid reason with an explanation to justify why you are saying no so that they understand you prioritized work and it can't be avoided.
Keep a check on your tone and use positive language and explain to them your pointers for saying No.
Offer them an alternative solution for their issue as it’s always better to propose an alternative option to them.
Show understanding and acknowledgment and know why they are giving you that work, as directly saying "No" sounds rude.
Ask for help from your co-workers or teammates to show them you care about the work and you are serious about it.
Suggest distributing and sharing the load that demonstrates your ability to shine and sympathy.
Don’t include your personal life and professional life to maintain objectivity and avoid emotional responses.