With all the recent articles on how closing apps from multitasking is more harmful than helpful when it comes to finally closing backgrounding apps there usually so many you get repetitive strain injury. 
Swipe Home can help you with this by allowing a quick swipe up on the home card when in backgrounding mode that it kills all open apps and if you flick again it will respring your device for you as well. 

This tweak is free from the BigBoss repo.