A quick summary of what i've gathered from my hre campaign:

- strike hard, strike fast - you need to clean up the rebel territories around you asap and break the back of at least one of your neighbours.

- damn the pope - he can't do much to hurt you

- spam cavalry - it works very well against the infantry heavy armies of most of your enemies (barring the polish)

- Convert some of your castles to cities - you need the income