Well it is hard to say what will be one winning strategy for all factions during long campaign, but usually you can figure out best strategy for opening moves.
For example, as Venice I always trade Iraklion for Bologna with HRE first turn. They demand no more than 1000 gold and you have solid centralized territory, which in my opinion is great starting strategy. Also by doing this you can delay war with Byzantine for a few turns – enough to kill Milan and pretty much secure your early position in the game.
The general rules are:
Don’t fight too many wars before you have solid economy
When you expand think if you can easily defend new territories
Play pope game as a catholic faction and use crusades / jihads to you advantage.
As Turks/Russia/Egypt – be prepared for Mongol an Timurid invasion because they can easily ruin well going campaign in just a few turns
And above all – do what it takes to have fun, be creative and experiment freely because M2TW is such a deep and beautiful game !