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, which can be useful anyway, 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. Just get as many as possible for yourself.
Note: I suggest keeping a first aid kit in your car, as well.
The phone numbers of your usual vet, an alternate vet in the same area, the closest emergency clinc, and the pet poison control center. Also, your pets medical history, a list of medications that he/she takes, and a good pet first aid book.
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 of dogs quickly, etc.), Foldable Elizabethan Collar, Piller, Syringes (several sizes, some with needles), Kelly's, Suture with an attached curved needle, Enema Bulb, Penlight or Small Flashlight, Zippered Bags for trash, and Empty Pill Bottles to put things in ~ from samples to used needles.
Peroxide, Alcohol, Betadine, and Natural Aloe Concentrate.
Antibiotics ~ Topical Cream, Opthamalic Cream, Tablets, and Multi-Biotic injectable - Which will actually be kept elsewhere, but I wanted to include it to have in the house.
Activated Charcoal, Vinegar, Salt, Olive Oil, Vaseline, Preparation-H, Ipecac (This is no longer sold, I don't think, but I left it, in case you have some.) Wet tail medication of choice, Pepcid, Electrolyte Powder, Acidophilus Capsules, Bottle With Dropper with Chlorine, Benedryl, Kwik Stop or other Styptic Powder, Epinephrine, Xylocaine, Vetericyn, Antibacterial/Fungal Ear Powder (I highly suggest having both Medicated Ear Fresh Ear Powder & Zymox, or other similar product for the second.), Eye Wash with Boric Acid, Valerian or prescription tranquilizer, Bach's Rescue Remedy, and Instant Coffee. (with caffeine!)
An Extra Leash, 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.
Be sure that you leave as many items in their original boxes/containers, with directions included, as possible. If something is moved to a container of your own, be sure to label it boldly ~ putting clear packing tape over the label to preserve it, and add any directions or warnings that it should have with it.
And please, don't forget a supply of whatever medication/s your dogs takes on a regular, or even semi-regular basis! Remember to rotate/replace medications and supplies regularly. Some, more often than others.