Although it sounds like a pretty obvious tip, it is really important to pay more attention on protecting your camera lens. If you put the lens on a pocket without proper protection, the lens tends to gather dust and various other micro-sized particles. These particles can get into the camera sensor apart from the back of the camera’s lens. Eventually, the camera will produce photos with annoying spots. If the lens is too dirty, it will even prevent from auto focusing.
Fixing such dirty lens can be done using a professional lens cleaning kit only. Don’t use random tools such as ordinary pieces of cloth and brushes that are made for other purposes.
More importantly, you should prevent your camera lens from getting dirty; this is exactly why you must use a camera lens case.