Adopt a vegan diet. Stop eating animal products (e.g. meat, dairy, eggs, fish, shellfish, honey).
Don't hunt or fish.
Don't buy products made of animal products (e.g. leather, wool, silk, feathers, fur, ivory, pearls).
Don't buy pets or any kind of animals.
Don't abandon pets.
If you are already vegan, try to find some time for persuading as many people as possible to become vegan, too. As an introvert, I don't feel comfortable talking to strangers directly, but I try doing my part by walking in my neighborhood and leaving leaflets into mailboxes with basic facts on veganism.
Don't buy products that contain real fur (for obvious reasons) or anything else that resembles it (i.e. fake fur) because it's hard to distinguish the latter from the former without analyzing samples in a laboratory. It's very important to avoid any risk, no matter how low, of financing fur farms.
Don't buy products containing carmine (E120), a bright-red pigment obtained from carminic acid, which is extracted from cochineal insects. It takes about 155,000 cochineal insects to make one kilogram of carmine. Natural carmine is predominant on the market because synthetic carmine is complex and expensive to produce. Carmine is used as colorant in food, cosmetics, drugs and textiles.
Don't use sharp metal spikes for bird control.
Don't visit circuses where animals are kept in captivity against their own will and forced to perform; circuses that don't include animal acts, on the other hand, are perfectly fine.