A very cool trick is the two ball flash start behind the back, which I've found most people don't have too much trouble learning. Instead of starting with one ball being thrown over your shoulder, you throw _two_ balls from that hand. The little snap of the wrist is what makes the balls come down non-simultaneously, giving you the chance to get the third ball up and begin your cascade.
Naturally, the logical extension of this is to start by throwing all 3 balls behind the back. Hold the balls in a triangle in your hand (I hold them with one of the triangle points directly under my index finger; YMMV). Toss them so that they separate slightly. Grab the two lower balls on the way up, then quickly start your cascade (or whatever). I can do this most of the time, and seems to impress non-jugglers.
Actually, there are two extensions.
First, during a normal cascade, catch 2 balls in one hand, flip them both behind the back, and keep going. This can be smoothed out by passing the other ball across the front in a low toss while passing the two behind in high tosses. (It also frees the hand on the side the balls are coming down on.) This seems to impress a few jugglers as well, and isn't that hard to do.
Second, which is an extension of the first, is to catch all three in one hand, and restart behind the back. This is tricky to get smooth, and no, I can't quite do it consistently yet.
I like to do this trick 'on the fly' ie from a cascade not as a start. It looks really good this way:
catch in the right hand (say) from the cascade and hold this ball. Make a sweeping motion with the right hand (like a reverse chop) to the middle and out over the next ball thrown from the left hand (in normal cascade timing). Snatch down smoothly on the second ball, giving you two in the right hand. Do the same for the third ball (trickier if not much room but you can kind of make a pocket for it) and continue the sweep (which is going to your right at about eye level or higher) out and around behind your back to make a three-ball either over the shoulder or (easier IMHO) behind the back, coming up under the left arm. You then have a three ball start...
When practising outside this trick more than any other gets me spontaneous applause and comments.
And here's my four ball extension:
In a four ball multiplex, instead of making a normal "double throw", throw two balls behind the back so that they split and each ball goes over a shoulder: if you get the timing right your then into a synchronous 4 ball pattern. I think it looks quite nice...... Have fun!
I find it easier to hold the three balls in a line. Then there is more separation between the lowest ball which I catch first and the highest. This gives a little more time to get the pattern started before the highest ball comes down.
Most people try to reach *way* across the back and lean *way* over to the other side. This doesn't look good and doesn't yield good results. Here's how I've learned to do it. Stand straight (sounds like mom right?) and (if you are comfortable with your arm position that you use to throw across the back) aim for a particular spot (your shoulder blade, or your ear) and try to hit it. I aim for either my ear or the back of my head. Something about the arm position has made me never hit the back of my head (or is it that I can't aim worth a damn?).
Now, if you are NOT comfortable with your arm position for the throw, here's my favorite position: To get the feel, stand with your arm hanging straight down the side of your leg. Now press your finger tips into the side of your leg and run your hand up your side (thigh, hip, ribs) to your armpit. That is the arm position. To do the throw, take the ball back slightly behind your and lift the arm (in the aforementioned way) and toss it towards the back of your head. It will fly either over or to the side of your head and come down right in the perfect spot.
Now all you have to do is learn the 5ball half shower and you're ready to do Bobby May's 5ball-half-shower-over-the-shoulder-trick (Peter Davison and some punky juniors also do this impossible trick). This is the hand/arm position for that trick.
A nice way to practice behind the back throws is to just use one ball. The first throw from the right hand should peak out to the left of, in front of, and a foot or 2 above your left shoulder. As it reaches the top of its trajectory, drag it down with your left hand, sort of a slow motion snatch. Then just swing your hand down and behind your back & make another throw.
It you get this going right it flows quite smoothly. There should be no sound of the ball hitting the hand. Also the hand doesn't actually close around the ball, it just cups it.
Another poster mentioned catching 2 balls in 1 hand, and then throwing these behind the back. One variation on this is to catch the second ball under your left arm, with your right hand. Then throw the 2 balls over your left shoulder. This is a good way of pausing during a 3-ball routine.
Another, perhaps not so natural extension, of throwing 3 balls behind the back follows:
While juggling three balls in a cascade, left hand held behind your back, reaching over to the right. (I don't know if this trick has a name. It requires some stretching so, lets call it a Stretch for the moment)
Anyway, juggle Stretch to the right, catch all balls in your right hand, throw them up behind your back. Catch them in a Stretch to the right and resume juggling.
And to make things symmetrical, flash from the right Stretch, switch hands and catch in a left Stretch, repeat ... (I can't do it this way yet.)
The Stretch is a really nice trick, but requires some practice. And to flash from l/r Stretch to the opposite side takes even more practice.