All good tips from Gilrandir - especially keeping a big army in a second-line province. If you keep your border provinces lightly garrisoned, the AI seems less inclined to attack, and when it does, it uses a smaller army so you can either fight or retreat and then counter-attack next turn.
I also find the Germans/HRE one of the more annoying early-game factions. I seem to spend far too long with just spear/archer armies, with way too many urban militia, and never enough money But Swabian swords are worth getting if you can afford them
As for battlefield tactics - practice, practice, practice.... and don't forget the pause key, too....