Something that hasn't been mentioned is how important morale is. When I'm advancing I am constantly rotating fresh high morale troops from the rear with my low morale troops on the front. Sending my low morale troops to 100% morale provinces.
Also. When moving troops across the water I transport them in my navy's ships. This protects them from losing morale.
So in short, Always keep an eye on the morale of your troops and rotate them out when they gets close to 50%. I like to keep them at at least 75% and up.