There are a few things that can "kill" your camera equipment in both function and value.
The first "killer" is DUST. While keeping your camera & lenses away from dirt and grime sounds like common sense, it's not always the case. Even if you avoid specifically dusty areas, it is inevitable dust will creep into every crevasse it can. The most important part to keep dust out of is your digital camera's sensor. Some ways to prevent this (as much as possible) are: If there's no lens on your D-SLR, it better have a body cap on it! Always keep the internal parts of your camera protected. It's best when changing lenses to do it when the camera is off and to hold the mount slightly downward as you're changing. Also, if your other pieces of equipment are dusty (like your lens) it can easily transfer to your image sensor. So, also keep everything as dirt & dust free as possible.
You can purchase compressed air cans which give a nice swift blow of air onto whatever you're pointing it at. There are also bulb blowers which are easier to transport in a camera bag, are inexpensive, and are an easy way to keep those nasty little particles away.
Another "killer" is FUNGUS. Fungi like to creep into lenses when you least expect it. On normal fungus levels, it is very hard to spot unless you know exactly what you're looking for. But the thing about fungus is, it doesn't stop growing. Over time it will etch the glass in a lens to the point of no return. Fungi is especially a problem in humid climates and I don't just mean the rainforest. If you live near a body of water, or a place where it rains on a fairly regular basis, your equipment is susceptible.
In addition to keeping your equipment properly stored, I suggest silica packs. These can be purchased in large sizes that can be re-activated by cooking it in the oven (best for larger spaces or multiple pieces of equipment), or for a temporary (& smaller space), the little packs that come in shoe boxes can be thrown into your camera bag as well. I also use these in my print boxes for preserving old photographs.