I earn an income from a regular full-time job, but very occasionally
(every few years) I also earn additional income as a screenwriter.  In
the past, I have tax-deducted screenwriting-related business expenses
only within the tax years in which I earned screenwriting income. 
During years where no writing income was generated, I kept records of
all writing expenses but did not claim them as deductions.

This year (2006), I again will be earning money from my writing, the
first writing-related income since 2003.  My question: can I deduct
not only 2006 writing expenses, but also writing expenses incurred in
2005 and 2004?  In other words, are such expenses accumulative, a kind
of multi-year loss which can be retroactively deducted once income finally is
generated?