Once in a while it happens here. I guess if it is sort of a constant/chronic thing it is time to rethink. I like "you need to do xyz please" better than "would you please do xyz." I start all those directives with "you need". If it isn't a request but a directive that needs to be clear.
I also tie things together, like, "you need to do xyz before you have your snack or turn on the tv or whatever they want to do." When my dd was really balking about practicing her instrument I would wait until a half an hour before her favorite tv show and say all enthusiastically, "hey! You have just enough time to practice if you want to watch Arthur". It was amazing how fast she could hustle and get it done lol.
And apparently I have some sort of "mean voice." That usually goes something like, "this needs to be done right now and if I have to do it then xyz (with xyz being something they won't like)."