In several campaigns I've played as the HRE I get to a point where my empire is stable and I have a good income. I start preping for a crusade and one of my allies (spain, italy, hungary, poland) goes to war with me and none of my allies stay on my side. So instead of going on a crusade my trade routes fall apart and I'm forced to abandon my crusade and defend the empire on at least three fronts . Any ideas on how to peacefully avoid this?

A couple questions of diplomatic regard:

What are good ways to make peace? I can't ever get the AI to cease-fire.
What are good ways to encourage an alliance?

I've assumed it's a good idea to get princesses from other nations to marry my heirs so that they can get an early start on producing offspring. If so whats a good way to convince nations to marry off their princesses?

thanks,
fredrick