I have 3 girls and they're all pretty good with manners, but it's an ongoing learning experience
Under normal circumstances (if they're not tired, hungry or sick) I typically push for good manners. I don't make a war of it but I remind them by asking them not to "sing it" or to "speak like a robot" (my 5 yr old dd responds well to that one), I'll even fake a proper English accent and repeat their request. Sometimes I respond with a "Miss B, did you say you would enjoy a cold glass of chocolate milk right about now?" (to really flower that one up, stand up and hold your hands behind your back and bend forward a bit) and that usually catches her attention for a better request. And then there's the lift of my eyebrows, which they get immediately. I make it fun and light.
When I'm tired (hungry, sick or it's after 7pm) I'll make a statement right back to a request like this: "I want chocolate milk!" "That's nice, I want to take a nap and then go for a walk." Although this doesn't work well on the younger one