Trying to “clean out” your tray is not a habit you want to get
into. It’s pointless, and besides, you can never win - as soon
as you run another app, in the tray it’ll go. It’s like the
world’s worst game of Whac-A-Mole. Instead, learn to see the
tray as a “recent apps” area. If you’re in the middle of one
task - say, writing an email - and you need to switch to something
for a second — say, looking up a spelling — then the tray is
your friend. But once you’re done with that, you’re done.