One of the things that Dr Kevin Leman suggests in his book, Have a New Kid by Friday, is to say things to your children once and then leave it. He postulates that they are intelligent humans and can process what you have said. Assuming you made sure you had their attention before you said your piece, leave it at that. Saying it more than once is telling them you didn't really mean it the first time, or that they weren't really sharp enough to understand with only one chance.
The practicality of this is awkward, depending on the age of your child, the request and where you are, but I do think there are lots of opportunities for us to use this every day. I know we've tried it a little at our place, but I think the jury is still out on how well we are doing.
If you would like a FREE copy of Have a New Kid by Friday, leave me a comment and I will send it to you. I have other Dr Leman books I am giving away for free, so go to this post and see what to do and what is available.