The first thing is to protect your camera lens against moisture. Keep the lens in a dry place, preferably in a bag specially designed for lenses.
Always buy quality lenses because you might think that you have saved money by buying a cheaper lens, but in the long run this will cost you a lot more.
Never leave the lens without its cover and only remove it when you are using the lens. This will prevent breakage.
You need to invest in an ultra violet light filter which should be attached to the front of the lens. This filter prevents dust and other objects from damaging the lens and also reduces the amount of ultra violet light entering the camera.
You can also install a lens hood to prevent flaring and other aberrations due to the light entering the lens.
Use a microfiber cloth and lens cleaning solution to clean your lens on a regular basis. It is imperative that you keep the lens clean even when not in use for several days as dust particles need to be wiped off for better picture quality. Most scratches occur because of dust, so if you keep the lens dust free there is a good chance you will manage to avoid small scratches on your lens.
In order to clean your lens thoroughly, you can use a small manual air brush to remove dust which is hard to get off. Be careful when using a cloth to rub the lens' surface, else you may end up scratching it yourself.