If one is going to ask for an upgrade, your best bet is to become friends with that CM. Form a bond with them. Talk to them, pay attention to them, be friendly, polite, patient, and most importantly, enthusiastic. When you ask, do it nicely, and let them know that you understand if it isn't possible, or that its okay if the room isnt ready yet. If it isn't possible, don't be dissapointed. Remember, just because the CM doesn't look up anything or go to the back, doesn't mean that they didn't try. They might already know from a previous guest that no upgrades are available.
It should also be noted that you might not be able to get an upgrade, and then 5 minutes later, the person next to you does get one. This is because they were pre-blocked for the upgrade days before you arrived.