There have been a number of threads on this over the years. Some people have said that they always write in the comments field that an upgrade is appreciated or identify why an upgrade might be appreciated (due to a special occasion). I feel that there are enough people doing this that the hotel staff just seems to ignore the comments. My advice is to book the room you want.
http://www.flyertalk.com/forum/hilto...ng-online.html
http://www.flyertalk.com/forum/hilto...upgrade-5.html
http://www.flyertalk.com/forum/hilto...ond-vip-7.html
http://www.flyertalk.com/forum/hilto...-upgraded.html
http://www.flyertalk.com/forum/hilto...s-section.html
http://www.flyertalk.com/forum/22352892-post14.html
http://www.flyertalk.com/forum/hilto...-upgraded.html