It is actually quite simple, just press the Power Key Located on the left hand side of the top of the phone. Now you can carry the phone in your pocket, or clean the screen with activating anything. This mode is also handy if you want to save the battery.
You can actually change how long the phone waits before it goes to sleep automatically. Just depress the
Menu => Settings => Display => Screen timeout
But to actually protect the phone, you should consider carrying it in a case. There are so many to choose from.

Or you can use a silicone case