The second thing I've noticed, is that most people tend not to throw high enough. I can't stress enough that learning to control a 20+ foot throw will help greatly for the higher numbers. If you can juggle 4 in a shower all with 20 foot throws it will much easier to learn 5.
The third thing applies to any juggling trick, PRACTICE! Even five minutes each day on a particular trick will decrease your learning time.
4 Balls Most books I've seen suggest starting with 3 in the right and 1 in the left. I start with 2 in each because it teaches you to toss those first 2 quickly so you have time to throw the other 2 over. The pattern is easy to guess, it's the same as before. Just throw a bit higher and a bit faster. Not what you wanted to hear is it? I'm afraid I don't know any sure-fire method for quickly learning 4, except for PRACTICE.
5 Balls Again, some books say 4 in the right and 1 in the left. Again, I say 3 in the right and 2 in the left. If you are serious about learning 5, I'll assume your 4 pattern is SOLID. I would recommend practicing 4 with fairly high throws, as I said in the general tips section. It will take a good amount of practice to juggle 5 consistently, perhaps more time than it took to learn 5 in the cascade. :-(
6 Balls I'm still in the practicing stage on this one. I can usually get 10-12 catches after some warming up. I start with 3 in each hand so those first 3 are thrown as fast and as high as possible while still maintaining control. If I can get those first 3 out the way I want them, the rest seem to come easily.