I have no Idea if anyone has come across this handy little tip before but I certainly have. It goes like this. You sign a right of passage agreement with another country. Of course this allows you to march any units onto their land. Once you have done this you can flood their territory with workers and then you can MODIFY THEIR TERRAIN IMPROVEMENTS! Now the computer will eventually change the improvements back again, but it can really effect their economy. Some modifications might be: mines to irrigation to slow production for some turns, or starve the populations of cities with irrigation to mines. The computer simply alows this to occur with no objection.