Sorcha--just foster something you know you don't want to live with. My parents fostered a litter of six mixed-pointer puppies a few years ago. The mother was a sweet but utterly brainless animal and they figured it wouldn't be as hard to give them up if they thought they might grow up to be hyperactive canine dunces. ;-)

I couldn't foster kittens, though. I can't pass up a cat. I fostered a big, brawny, ill-tempered calico for work once and had a Hell of a time giving her away even though she tried to bite me every time I made her do something she didn't want to do (and she didn't want to do very much, believe me), like get off my lap or stop scratching the door jamb. I never saw an ugly cat or a kitten that wasn't unbearably cute, no matter how bedraggled.