Half the time when I close I come back and its still there, especially on SP3 with the larger screen swiping top to bottom to close is suboptimal. They should just include the X on all when you swipe from the top edge so you can close quickly and positively.
For now ... the better way is to go to, the start screen, swipe in/out from the left to show the apps list the. Drag the one you want to close to the bottom. Still inefficient as reaching around your back to scratch your elbow though.
Killing open apps in task manager is another option but that's not really finger friendly.
Id suggest maybe using gesture studio with Kinect so you can give apps the finger to close