Prestige is like experience points in other games. Prestige shows a general ranking of you vs. everyone else on the server. Prestige does not show how good you are at fighting though! Someone may have a low prestige but be good in combat vs. other players. Someone else may have high prestige but be a newbie!
You earn prestige for building your city, training your army, researching in the academy, and fighting NPCs. Generally speaking if you are constantly building and improving your cities you will earn prestige.
There used to be some tricks like building 1 troop at a time or sending a few trips to die at the hands of a barbarian. Most of these bugs have been fixed though.
I do still build batches of troops because you can queue them up. If I have a level 4 barracks for example I can queue up 4 batches of 100 archers. That way I get small boosts of prestige, I am always adding troops to my army, etc. If I trained 400 at a time it would take 4x as long to have those reinforcements available. Plus I think building small batches is good for prestige.
Finally one theory out there is prestige is based partially on your rank. If you have a rank that allows you to have 5 cities but you only build in 2 then you might not earn prestige as fast. Building in multiple cities at once seems to be the best way to earn prestige fast.
Do you have more tips on earning prestige?