[Lingo] spaces in japanese writing

[steven smith <sjs@example.com>]

The woman I've been working with on my Japanese said
something the other day that surprised me.  She said there
should be spaces between some of the words, but she couldn't
tell me if there was a rule or a convention for this -- only
that some of the words should have spaces between.  This was
the first time I'd heard this and was surprised.  She is a
native Japanese but makes no claim about being a teacher.

I tend to use spaces because I'm still learning and it helps
me separate the words so I have some chance of reading what
I've written.  I received a note from a friend (email) and
out of about200 characters there were 3 spaces.

So -- is there a rule/convention for this??  When should you
use spaces when writing Japanese?

Thanks
Steve S.