Make the reservation your normal way.
A week or so before the trip call the hotel and ask to speak to either Guest Services or the GM.
Say that you'd like to do something special during your trip for a special occasion and ask about what they can do - bottle of Champagne in the room?, flowers?, etc. Don't ask for anything free.
If you're nice and pleasant and chatty it's quite possible that you'll have a very nice room upgrade upon your arrival - but don't count on it.