We use several different yard signs and post them several days prior to a cleaning.  Black lettering with yellow background work well for us.

This strategy gets the attention of peoply driving by.  It causes them to focus on a dirty roof a few days prior to cleaning.  Then, when we are done, they are more aware of the before and after affects of the roof cleaning process.

3 Days in advance, we put a sign that says "Roof Cleaning in 3 Days", then the next day, we change the number to 2  etc.  This count down seems to draw attention to the house very well.  We are also purchasing Banner Flags.  We are also having signs made that say
"Free Demonstrations and Free Estimates".