Bob's suggestion ((multi-finger up-swipe, long-press on an icon displayed at the bottom of the screen to bring-up the "minus" sign, then tap to close apps you aren't using. They can be re-opened from the home screen later) does't work for me.

I'm running the latest iOS7 on an iPhone 4S. When I up swipe (multiple times or not) all I get is the Control Center screen, which has only four icons at the bottom: Flashlight, Timer, Calculator and Camera.