That's out of my scope.
“No, I’m sorry, I’m not an expert in XYZ, but I’ve worked with so-and-so, who is. Let me introduce you.”
This is great if a client asks you to do work outside your normal scope.
It's not you. It's me.
“Unfortunately, I don’t think my company is the best fit. Here's someone who might be.”
If you’re targeting different sized clients in different industries, this way to say no is great.
Now's not a good time.
“I would enjoy working with you, but I’m on deadline with two projects and would like to be able to focus on yours.”
If you’re genuinely slammed and don’t have time for a project, this is a great way to say no. It also works if you just don’t want to do the type of work the client needs or if you don’t feel like working with that client.
“My rate for that package is [double the normal rate].”
If you need work but foresee working with a client to be a pain, charge an "inconvenience" fee. If they say no, you’re off the hook. And if they say yes, you make bank.