how to force some line spacing

in a set of exercises (which i'm representing with a 
simple ordered list), i'd like to force some space between
each question to give the students room to write their
answer.

  what's the most elegant way to leave whatever number
of spacing lines i want?  (sometimes i'll want just a couple,
sometimes considerably more.)

  any hints?

rday

toolchain:
  xsltproc: DB -> FO
  FOP: FO -> PDF