Say no with grace.

Count to three before saying no.

Provide an explanation as to why you are saying no.
For example, you could say that you are currently too busy to take on any more projects, but your schedule is open at the end of the month.

Reply with, “Let me check my calendar and get back to you.”
This will give you time to look through your schedule and effectively evaluate how much free time you have.

Use automated email bouncebacks.
If you know you are going to be busy for a certain period of time, set an automated email bounceback to inform those who email you of your unavailability.

Reply with, “Yes, what tasks should I deprioritize?”
If a senior leader is asking you to take on more work than you can reasonably handle, ask them for their advice on which tasks you should set aside for later.

Say it with humor.

Use the words, “You are welcome to X. I am willing to Y.”
For example, you can say, “You are welcome to borrow my car. I am willing to make sure the keys are here for you,” and you are also making it clear that you will not drive them.

Suggest someone else for the task.