Have a chair ready. Plonk your planning and data sheets on it for them to have a look at.
Make sure you prove that you know your class and that what you are doing is what they need to progress. Don't be afraid to change a lesson and teach what they need, if things don't work out as planned. Evidence that you know them through: Planning, targets, highlighted FSM, summer born, boy/girl, SEN, G&T.
Make clear where you are in a unit with your planning and provide them with a general overview of the term/unit.
Have your books marked.
Use good questioning, differentiation, A4L strategies, mini-plenaries - basically, do what you probably do anyway, but make it so idiot-proof and obvious that anybody walking in would know straight away what you are doing and what the point of your lesson is.
Prepare your children at least a little. It's equally weird for them to have strange people walk in, sit in a seat and scribble notes...then wander around and ask them about their books. Mine are used to people coming in,...they aren't used to these people failing to interact with them as usual.
Don't overplan. Don't stay at school for hours. Don't fall asleep in your own lesson obs, because you stayed up cutting out resources all night. Don't try an all singing, all dancing lesson, if that's not how your class usually works.
Good luck. 