Preparing for an emergency is always good advice.
The first step: Program your veterinarian's phone number into your cell phone along with the number for your local veterinary emergency hospital.
Next: Assemble a first-aid kit.
Basic supplies include: adhesive tape, antihistamines (such as Benadryl), antiseptic (such as Betadine), cotton (balls, swabs and rolls), corn starch or styptic powder (such as Kwik Stop), Karo syrup, tweezers, hydrogen peroxide, scissors, sterile gauze (pads and rolls), pet thermometer, antibiotic ointment, anti-diarrheal (such as Pepto-Bismol), clear plastic cling wrap (such as Saran Wrap), activated charcoal and a water-based lubricating jelly (such as K-Y). Add a soft fabric muzzle (for dogs) or a restraint bag (for cats), since an animal in pain may lash out.
Consult with your regular veterinarian regarding medication dosages.
You can buy a ready-made kit from a pet retailer. Hunting-dog outfitters have selections of kits in durable containers -- no surprise, since an injury to a working dog may happen far away from a veterinarian.
If you would like to purchase a great reference guide, Amy Shojai's "The First-Aid Companion for Dogs and Cats" (Rodale Books, $20) is one of the best books on pet first aid around.