I never asked for an upgrade except in London if someone sharing the room with me not when I'm alone ,most of the time they upgrade me especially in Asia and Middle East ..
by the way one time in China Marriott hotel in Shanghai I was alone they upgrade me to a huge suite I come back and ask for normal room because I was going for business and I just need the bed ..
If the upgrade is something very important to you show them that a bigger room Is empty the time of your booking in a nice way they well not say no ..
Upgrades in my opinion ..
USA and Canada.. hardest
Eeurope and Uk.. hard
Australia.. normal never say no by the way but you should wait for a room ..
Asia.. easy except Male
South America.. ?? Never been there
good luck