In the last game I played as England, some time ago, I established my favorite four-region front: Friesland, Lorraine, Burgundy, and Provence. The HRA occupied the regions bordering them to the east.

I noticed that the HRE's zeal in those regions was unusually high, and they were slowly but surely recovering from an earlier stomping. I was waging war elsewhere and just wanted to maintain a strong border in these four regions. So, I built a Chapter House in Friesland, started crusades there, and used them to thin out the regions bordering mine as they made their way south and east. This worked well, though admittedly it seemed a bit of a cheesy method.

Generally, I don't use crusades as much as I should, an exception being when I play as the Sicilians. Again, passing through the Papal, Italian, and Hungarian regions adds manpower to my crusader army while weakening those that border my lands.