An interesting article on LockerGnome suggests that 20 commonly used words and phrases will trigger junk email filters. They are:

amazing
cancel at any time
check or money order
click here
congratulations
dear friend
e-mail marketing
for only ($)
for free
great offer
guarantee
increase sales
order now
promise you
risk free
special promotion
this is not spam
to be removed
unsubscribe
winner