It happens to me all the time. People want you to do stuff you can't/don't want to/can't afford to do.
So say this:
I really love having you as a client, but I’m going to have to say no because…
- With our pricing structure the way it is, I’d be losing money. If you really want me on this new job, we’ll have to sort out a new fee we’re both happy with.
- We don’t have the resources for that, but what we can do for you is….
- That project falls outside of my area of expertise, what you need is a person who does X. Here are some recommendations...
- My client load is really chock-full right now and I don’t think I’d be able to give your project the attention it deserves.
- I don’t think my skills are a good match for what you want for this task
It’s super important to emphasise what you can do, not what you can’t. Hopefully your client will understand (not always as some people do often suck).
Don’t undervalue yourself & don’t do anything you’re not comfortable with.
That way, no stress-balls required.