I alway check the Carnival website every week to see if the price changes and then contact the agent immediately. You're right, my agent handles many other cruise lines and tries to monitor the changes, but a little help from me doesn't harm it at all. I know she means well and tries her best.
I use to book the Balcony guarantee and hoped for an upgrade to something special, but that never happened. So I've went to picking out my own room where I know it's something I want. You read where occasionally someone gets upgraded to a suite, but I think you have to be very lucky for that to happen. Luck is not my strong suit.