1986 - Don't Do It
Melle Mel (a true pioneer in the realm of rap) and producer Van Silk get down in the streets and tell you to stay away from crack. This commercial, according to Melle Mel's website, won an Emmy.
Update: Or maybe it was a longer commercial with the same guys, as indicated at the link
http://www.popfactor.com/tmftml/archives/002368.html

1988 - Just Say No
No, The Golden Girls do not have any sort of influence over the youth of America. No, Mister 1988 Well-Meaning Public Servants, putting grandmothers up on the screen to tell us to Just Say No will have no impact except to make us wonder if The Golden Girls ever sneaked something themselves in the name of becoming Hip Old Ladies.

1987 - Crack Is Wack
From the very heart of the drug wars, the members of the group Turning Point (about whom I can find nothing in a brief Google search) tell the kids to Just Say No, and irony is not involved.

1989 - Girls' Club of America
Remember girls, don't have dreams or fantasies when you're a little kid too young to actually get a job, because fantasies make you so poor you'll go on welfare.

1993 - Open Letter to Janet Reno
The background to this item, which I would say counts as a genuine public service announcement, is best explained at the following website:
http://reason.com/Penn.shtml
Penn Jillette, with assistance from Teller, explains a thing or two to Janet Reno.