Before putting your iPhone to sleep, press the home button. This ensures that all apps are turned off, and it saves you battery life in the process. If you simply put the phone to sleep without pressing home, you may have some apps running in the background and reduce your battery life.
If you happen to drip your iPhone in water, avoid turning it on; this could short circuit your phone. You can dry it out by gently towel drying it or putting it inside a Ziploc bag filled with dry, white rice overnight. Avoid using a hair dryer on it as well, as this can cause water to seep deeper into the phone.