Easier said than done - but Keep calm........ in any situation as they like to push your buttons and see how far they can push ou in any situation. I dont always keep calm but try my best...
Also a very important things with toddlers, mean what you say and say what you mean - if you threaten or promise something - follow through with it otherwise next time your words will be meaningless.
Think before you say something as once you say no - you shouldnt go back on this or next time they will just nag you for longer to try and get their own way.
Also think about your answer if they want to do somethingyou would rather not - is it worht the trouble / tantrum? Do they want to wear their wellies with their shorts- does it matter? might not be what you want them to wear but it is not really important...