Another useful technique is called “broken record”, where you basically keep repeating the same answer. If someone is persistent and keeps repeating requests, its tempting to keep finding new ways of saying no. This gradually dilutes your response and makes you bring in excuses and apologies.
So if at the request from a colleague is to work a shift for them, you reply:-
“No, I can’t work that day”
“But I really need someone to cover for me”
“No, I can’t work that day” (Broken record)
“I’ve asked everyone else, you’re the only one who can help”
“No, I can’t work that day” (Broken record)
“Why not, you usually can help me out?”
“No, I can’t work that day” (Broken record)
The final response can be different, having been asked a question e.g. “That’s my business, and I still can’t work that day” – but as you build confidence it can be easier to stick to the broken record.