Regarding the back cinch -- It should not be cinched up tight by any means, but you shouldn't be able to see any daylight between the cinch and the horse's belly, when the horse is standing. It's not doing any good if it's loose and if it's too loose you run the risk of your horse getting a back leg caught up in there, which would be a huge wreck!
This is a good example:
http://i114.photobucket.com/albums/n.../backcinch.jpg