Four ways to say no nicely
Limit your services
If you're OK with fulfilling part of the favor, feel free to lend a helping hand. But be clear on what you are and aren't willing to do. For example, "I'd love to help you out by working your shift next Thursday, but you'll need to find someone else to cover Friday."
Recap the requestSoften the no-blow by first showing that you heard the appeal. Then close with a firm no.
Stick to your guns
When petitioners persist, just repeat your refusal. Giving a short, honest reason for declining may help—but only if you're comfortable disclosing it, of course. If they begin to bully, say no once more and walk away.
If you don't want to do something, a simple "No, I can't" is fine. You don't owe anyone an explanation or apology.