My own personal style is to give advancements for pre-campaign time in service:
1 advancement per two years of "civilian" service
1 advancement per year of military (Starfleet, IKN, RSN, Cardassian military, etc.)
1 advancement for 6 months of military wartime service
I prefer this mostly because I'm still a fan of the way FASA treated previous experience.
Davy Jones
"Frightened? My dear, you are looking at a man who has laughed in the face of death, sneered at doom, and chuckled at catastrophe! I was petrified."
-- The Wizard of Oz