Two of the things that can kill you the quickest if you become hopelessly lost in the wilderness are dehydration and injury. Right off the bat you'll want to consider contents such as water purifying tablets, a water filter and a metal bowl, so you have a strategy for collecting any water you come across and making sure it's safe for drinking. (Adding a pinch of salt per liter of water can help maintain electrolyte levels if food is scarce.) A first aid kit is also a must, and it shouldn't be some cheap affair with just gauze, a couple of adhesive bandages and a little bottle of antiseptic. If someone is injured out in the middle of nowhere, there's no quick ride to the ER in the cards, so be prepared to treat more serious wounds should they arise.
Next on the list is a knife. It's immensely important to have a decent survival knife along for the ride. They're incredibly useful tools that are able to serve a wide variety of uses; everything from helping to create a shelter and start a fire to catching and preparing food. Having a sharpening stone already on hand is smart, and a multipurpose utility tool can add even more versatility. For more on survival knife selection, use and safety, read How to Use a Survival Knife.
Another major item that should be included in your survival kit is a set of waterproof matches. Better yet? A lighter and a set of matches, all in watertight containers since even waterproof matches can wither given enough water exposure. Lighters help provide more efficient, reliable and longer-lasting lighting power, but stuff can still go wrong with them. Better to double up on something as important as this. A flashlight is also useful; not only can it help you see at night, it can help rescuers see you, too.
So now your thirst is slaked, you cuts are tended and your fire is blazing. It's time to start giving some serious thought as to how you're going to shelter and feed yourself, and more importantly, how you're going to get out of the wilderness and back home to your family on the double.