Adopt a Pet!
Check out: http://theshelterpetproject.org/ or http://www.aspca.org/adopt for profiles of pets in your area that desperately need a home.
Already have pets? Be sure to spay or neuter them.
Visit: http://www.humanesociety.org/issues/pet_overpopulation/tips/afford_spay_neuter.html to find a low cost spay/neuter program in your area.
Volunteer at your local shelter. So many shelters are understaffed and underfunded. You would be amazed at how valuable just a few hours out of your week are to shelters and their animals.
There are even organizations that enlist volunteers to help by just walking shelter and rescued dogs.
Visit: http://www.volunteermatch.org/ to find amazing volunteer opportunities in your community.
If you have a good-sized home and can financially swing it, why not consider becoming a foster home for pets?
Fostering saves the life of an animal and gives it the time it needs to find a good home. Foster animals are also much more likely to be adopted, since you will be able to vouch for their behavior and personality.
Contact your local shelter for more information.
Find out if your state allows the use of CO gas chambers to kill shelter animals. If they do, contact your legislators and ask that they be banned.
Also, find out if there are any laws against puppy mills in your state. Demand that puppy mills be shut down or, at a minimum, be inspected by the government so that the animals are treated more humanely.
Please be sure to follow Leviathan Project on Twitter and Facebook for current information on this issue and up-to-date ways to get involved: