I keep mine in a good quality gym-type bag because I've found that they can be pulled out from under things that fall on them more easily; as long as you pad towels around everything inside. (I'm from earthquake country, but it certainly isn't limited to that situation!)
I have tried to put the items in some type of order, and while I realize that some of the items listed are not available without a vet, they are what I keep in mine, and I do suggest that you get as many as possible for yourself.
Also, I always suggest keeping a first aid kit in your car, as well!
Tweezers, Scissors (two or three types including small nail scissors), Thermometer, Safety Pins (regular and horse blanket size/type), Splint/s, Nail Clippers, Pliers, Craftsman Multi-Snips (good for getting collars off dogs quickly, etc.), Foldable Elizabethan Collar, Piller, Syringes (several sizes, some with needles), Kelly's, Suture with an attached curved needle, Bulb, and Empty Pill Bottles to put things in ~ from samples to used needles.
Peroxide, Alcohol, Listerine, Betadine, and Natural Aloe Concentrate.
Antibiotics ~ Topical Cream, Opthamalic Cream, Tablets, and Multi-Biotic injectable.
Activated Charcoal, Vinegar, Salt, Olive Oil, Vaseline, Preparation-H, Ipecac, Wet tail medication of choice, Pepcid, Electrolyte Powder, Niacin, Echinacea Drops, Acidophilus Capsules, Bottle With Dropper with Chlorine, Pepto Bismol, Benedryl, Kwik Stop or other Styptic Powder/Stick, Epinephrine, Xylocaine ~ injectable, Antibacterial/Fungal Ear Powder (I highly suggest Medicated Ear Fresh Ear Powder), Eye Wash with Boric Acid, Valiums, Valerian, Bach's Rescue Remedy, and Instant Coffee. (with caffeine!)
Ace bandages, Vet-wraps, Paper Tape, Cloth tape, Duct Tape, Man's Ties ~ narrow variety, Cotton, Q-Tip's, Various sizes of Gauze Bandages, String, Towels and Wash Cloths, and a couple of Trash Bags With Closures.
And please don't forget a supply of whatever medications your dog/s take/s on a regular, or even semi-regular basis! That needs to be rotated/replaced regularly.