One approach I use is to say that although the 'idea' is good/interesting/brilliant I am not passionate enough about the project to be able to give it my all.

One approach I use is to say that although the 'idea' is good/interesting/brilliant I am not passionate enough about the project to be able to give it my all. On a related note, I think of the worthiness of ideas based on whether they are interesting enough to get me to work on them in my spare time even if it were for free.