You have two different questions.
One: What's the best way to make requests? I think chat is probably slightly better than calling, b/c they might be able to cut and paste your request.
Two: Is there a way to have consistent success with requests? No.
Because of #2, I try to keep my requests pretty generic. "Upper floor" is almost always in my list, plus maybe a general area of the resort. For example, at OKW-General, my request is usually "Upper floor with a water view." At SSR-Resort, it is "Upper floor, near pool." CCV it was "Upper Floor, away from elevator." For Jambo-Savannah, I'm either going to just take my chances or ask for Arusha.