I first learned to play from Pete Seeger's book and got stuck for years up-picking: the basic strum is

one: pick up on one of the top three strings
and:
two: strum down across four strings with the nails
and: pluck the fifth string with your thumb

the rhythm is boom tid-dy, boom tid-dy.

after that you add at one and a hammer on, pull off, or slide:

one: pick up
and: hammer (or pull off or slide)
two: brush down
and: thumb on 5th string

I got as far as doublethumbing: on the one and, instead of a rest or pause, the thumb would pick a note on the second, third, or fourth string; on two, instead of brushing down, you pick a string with your index finger and follow with another thumb on 5th string. I got pretty good at doublethumbing melodies, but I was still pretty much stuck with that plus guitar style picking and a rhumba rhythm I also learned from seeger and used for calypso music. I was playing almost exclusively solo at the time, primarily to back up my singing.

I tried frailing, following seeger's instructions, but it felt so unnatural that I didn't stick with it long. I finally learned a bit of three finger picking--it came fairly easily once I started accenting the first, third, fifth, and seventh note of a three-finger roll:

not 1-2-3-4-5-6-7-8- but
1-2-
3-4-
5-6-
7-8-.

I never got very fast, but it didn't matter: I was still playing solo (no wonder!) and still playing mostly to accompany singing.
It wasn't until about thirty years after I picked up the banjo that I started playing regularly with a guitar player and an autoharp player, both of whom also frailed the banjo, and the kind of songs we were playing cried out for frailing, so I started learning it. The basic frailing strum has the same boom tid-dy rhythm that up picking has, and is decorated in the same ways, hammering on, pulling off, sliding, and double thumbing, or more exactly, drop thumbing. The distinction is of vital importance: in up-picking doublethumbing, you use a pinching motion: finger up, thumb up, finger up, thumb up. Maintaining a solid rhythm is easy because your hand kind of rocks back and forth between the finger and the thumb actions.
but if you try to pick up with your thumb after picking down on a string with the nail of your index (or middle) finger, you'll find it damned near impossible to combine consistent rhythm with speed and accuracy or thumb placement on any but the fifth string. That's why it's important to make the distinction between double and drop thumbing: in clawhammer style as the finger picks down on a string, the thumb drops into place on the string it is going to sound (vital note--neither the finger nor the thumb provides the motion to pick strings; the motion comes from the arm, pivoting at the shoulder:

one: finger picks first (or 2nd, 3rd, or 4th string) as thumb drops into position behind second string
and: thumb picks second (or 3rd, 4th, or 5th) string as hand rises
two: finger picks string
and: thumb picks fifth string

I'm still not as fast drop thumbing as I am double thumbing, if you see the distinction, but I'm getting there. Another thirty years of practice ought to do it for me.

--seed