What is the best way I can help homeless cats, if I cannot adopt?

Can you foster any? Rescue groups thrive off of their foster homes, without them they can't do anything. You can foster a litter of young kittens before they are spayed/neutered (it sounds like a lot of work, but really it isn't that bad), you can even confine them into one room. If you can't do that tell everyone you know about spaying and neutering, and also about adopting.Tell them about the wonderful pets at the shelters and the harsh facts. Volunteer at a shelter or rescue, they would love your help.