I just ask. But I do it slyly...
"Ummm I'll have a can of coke zero, thanks".
Rather than, "I'll have coke zero...oh actually can I have the whole can?"
The only time I've ever not gotten it were if she/he didn't hear me, or if they were running out. Some airlines don't have cans also. From what I've seen Avianca is most bottles. They get Coke from a big 2 liter bottle and on the flight I was on last week coke zero from a 500ml bottle, which she then offered to me.