You might already know this little trick but if not, it's too good not to add to your toolkit of behaviour management. So imagine you need your child to do something you know they are going to be reluctant to do. How do you ask them?
It is surprising how more effective the second command is. You see 'thank you' denotes it's already done whereas please is more of a plead and pleads can be opposed more readily! If you add an air of 'I am just assuming you are going to do this' rather than anticipating you are going to engage in a battle of wills, this can also help.
And although at first it feels like an odd way to make a request, you soon get used to it with practice. Just a further not, it is important to get the tone right: authentic but firm and not at all sarcastic!