After a crusade, I destroy everything and give it away. Either to the Pope or a to a faction that is close to the original owners, just to cause trouble.

Then I just keep conquering and sacking and destroying until my army is dead and I am filthy rich.

Its basically just a profitable suicide squad...and so I only do it when I have an old general or two who won't be around much longer. If they are that old, I can't rely on them to lead a standing army for any significant amount of time...may as well put them to good use and let them die with honor.