Really easy & very daft too. This trick requires a loose T-shirt & it will get stretched so be sure to practise with an old tatty thing & not one of your nice convention T-shirts. Throw a ball up, stretch out your T-shirt & bounce the ball up off of it. Endless entertainment.
This may seem obvious but make sure your shirt is untucked before you begin!
Start juggling & make a High Throw straight up in front of you. Quickly reach down & grab the bottom of your T-shirt with both hands, stretch it out in front of you & lean back a little so that it forms a nice flat 'trampoline' style surface. At the point when the ball comes down & hits the trampoline you need to push the T-shirt up into the ball, the force of the ball alone is not enough to get any real height out of the bounce. Let go of your shirt then resume juggling.
Worth its own tutorial? Probably not.