Play with transitions: The transitions between "normal" juggling and multiplexing can give you some good ideas for tricks.
Invent new throws: By exploring variations of the most basic unit--the single multiplex throw--you may uncover some good potential for new material.
Stacked cascade: You can juggle more than three balls in a 3-ball cascade by replacing single balls with 2-ball stacks. Start with 4 balls and work your way up to 6.
Stacked fountain: Juggle a four-ball fountain and try to use stacked multiplexes. Of course there is space for eight balls here, but start with 5 and work your way up. For practice, try three in one hand, where one ball is a single and the other a stack. Then try 4 in one hand, both stacked. [Thanks to Simon Fox.]
4-ball split multiplex: Do a normal reverse cascade, except throw a split/different height multiplex throw every time from one hand (i.e., the right). Starting with 2 balls in each hand, throw a single ball from the left, then a split multiplex from the right. Repeat. Switch hands. Repeat . . .
Standard 5-ball multiplex: This pattern is based on continuous split/different height multiplexes in a reverse cascade pattern. Work up to it with the 4-ball pattern above. (Practice the multiplex in your weak hand, in particular.) This pattern is slower than you think. Take it easy.
Standard 7-ball multiplex: If you have mastered the 3-ball multiplex throw, practice the 7-ball multiplex pattern with 6--2 up/1 over from the right, 1 up/1 over from the left, repeat. (And then try the reverse.) The 7-ball multiplex pattern simply involves lots of practice and big hands.
The multiplex shower: Mike Hayes describes several variations of this pattern in the shower section of the "balls" help files. Another interesting variation can be found by ignoring one of his rules for showering 5 balls in a 3 pattern:
To juggle 5, start with 3 in the right, 2 in the left. Throw one up as normal, throw the next two together and pass the other 2 over. The idea is the same as before, you just have to follow two sets of 2 balls, and not confuse them [emphasis added] with each other or the single ball. --Mike HayesInstead of keeping the 2 two-ball units and 1 one-ball unit distinct, try merging them so that after your first 1-ball throw every throw involves 2 balls. You are thus switching the two balls that are thrown together, much like in a normal 5-ball multiplex cascade.
A more graceful transition can be (relatively) easily done by making the transition right after a normal split throw. Throw a single ball (keeping the extra one in your hand) instead of the next multiplex. This first solo ball should go between rather than over the 2 balls from the previous throw since you are switching from a reverse cascade to a cascade. [It is just as easy to switch to a normal 5-ball reverse cascade here by throwing over, except for the fact that most people's cascade pattern is much stronger.] There is a change of tempo from the multiplex pattern to the regular pattern, so you may have to rush a bit.
Once you've mastered the above, you can work on my favorite transition--a multiplex flash. Throw two multiplexes in quick succession, the second higher than the first. Throw the single ball left in your hand up to match the second height and if all goes well the balls will fall down into a cascade pattern. [The timing is odd, and I'm not sure exactly what's happening, but it does work. I have a siteswap that works, but I'm not sure it's what I do. I will try to post a better description when I figure it out.]