how do you juggle? not like moshi kame.
my best advice is to find 2 balls, or i suggest hackie sacks
then learn to juggle 2 balls with 1 hand...specifically column juggling
http://www.monkeysee.com/play/1187-juggling-two-balls-in-one-hand (http://www.monkeysee.com/play/1187-juggling-two-balls-in-one-hand)
Learn 2 ball columns, then to juggle them as low as possible.
Once you have that practice juggling just the ken on its own so you get a feel for how it rotates.
I guess you could practice stringless or with a long string at first then practice throwing lower and lower until you can do it with the usual string length.
Also with the big cup juggle (big cup - juggle - big cup) think, knees - no knees - knees i.e. don't bend you knees whist juggling but do on the catches