Please is the magic word!
I was at a children's party standing round talking to one of the dads while children played noisily around us. Children laughing, playing, running here and there, having fun. Groups of parents talking to each other.
While I was talking with one dad my child came up and said "Can I have some cheezles?" It was great that they were asking because some children would just eat what ever's there, but my child wanted to know which foods he can have, and which ones might be off limits (such as soft drink).
I kept talking to the dad as though my child hadn't spoken. The man was surprised and stopped talking to look at my child but I acted as though I'd heard nothing.
My child asked again "Can I have some cheezles?" I still said nothing and acted as though I couldn't hear him. By now the man was squirming a bit and thinking I am a very rude mother not acknowledging my child. I didn't let what this man was thinking of me change my tactic.
Then my child says "Can I please have some cheezles?"
"Yes," I beamed at my child, happy that he'd used his manners. My child smiled too, happy that I'd said yes and he'd remembered the magic word.
The man nearly fell over he was so surprised. He had thought I was being deliberately rude and couldn't understand why. When my child asked using the magic word he was stunned and amazed and no doubt a little (very) impressed.
My children know if they don't use their manners the answer is no, or quite often I won't answer at all.
If they say huh? I don't repeat myself. I wait for them to say pardon. If they ask nicely for something I don't let go until they say thank you, or will take it back until they say thank you.
They have learned manners must be used no matter what.