rotation in the garden

have realised what my problem is re rotating the garden beds in an
organised 
manner:

1: lots of brassicas. i seem to grow half brassicas & half other stuff!! 
(only slight exaggeration). this makes rotation difficult! in summer, lots

of solanacae (sp!) as well, of course, which have to be somewhere
different 
each season.

2: not everything comes out at the same time - the garden goes all year. 
(how is one to rotate in this circumstance?!) i try to keep a record &
then 
consider what was in each section of each bed - the entire bed isn't taken

as a whole because they're quite long, with perhaps up to 10 different 
things along the length - and only in the same family if it's worked out 
that way. if it hasn't worked out that way, i can't do it.

in a nutshell, if i rotate as best i can but still grow lots of brassica,
am 
i setting myself up for a disease disaster? i simply cannot think how to 
plan good rotations under this circumstance. with the brassica i take out 
the entire roots, so each location gets a rest - is that enough?

the whole family is now in love with chinese cabbage & pak choy, so if i
am 
setting up a problem, it is only going to get worse.

thanks for any help or ideas!
kylie