So, you've got the right name, you've done your bob or nod, now... What do you say!? Firstly, never speak unless spoken to. If you do have opportunity to engage in some right royal chit-chat, aim for anything within the public realm. Put simply, nothing private. "Did Your Majesty enjoy the wedding breakfast?" is ok. "Who's looking after the corgis?" is probably not.
Be yourself (to an extent); avoid using slang or coarse language, but don't be nervous and certainly don't try to put on an ultra-posh accent.