What would be a good reason to give to get out of doing a favor?

We all at one time encounter a request that we simply want to say no to but in a good way. Unfortunately we occasionally draw a blank and say yes when we wanted to say no.
I’m not giving an example as to what kind of favor. I’m just looking to collect a variety of excuses.
Please be creative. Don’t use ones, like my dog just died.
This is simply for fun. Figure some of you may enjoy having an extra excuse or two.