Just like your pets or babies, your iPhone can be permanently damaged if you leave it in a hot car. It can also happen if you leave it in the sun.
In iOS 9, Apple added a new feature that allows you to conserve your battery by putting your iPhone in low battery mode. You'll get a prompt when your battery percentage dips below 20 percent.
If you don't turn it on with the prompt, you can also find Low Power Mode by going to:
Settings > Battery > Low Power Mode and turn it ON.
It is a common misconception that closing your iPhone Apps will help conserve battery life. When you close your App it is taken out of RAM (short term memory) and when you reopen it it is put back into RAM. The App is generally not running in the background, but frozen until it is reloaded again.
According to this Mac 9 to 5 article:
I misplace my phone every now and then. Calling it usually helps me find it quickly, however, if the battery is dead that's another sad story.
The latest iOS 8 update has the ability to have your missing iPhone send its location to Apple just before the battery dies. It uses a cellular signal to identify the location of the iPhone, so it won't work if your iPhone was in airplane mode when you lost it. This new feature works for any device with iOS8 and also works on your iPad.