Hi M.,
There is an interesting book called " The Scientist in the Crib". The researchers who wrote it believe after observing the way children learn and think that the "terrible two's" are not so much about deliberately being naughty , rather, that they are making the massive realisation that they are an individual in very specific ways, eg, " mummy doesn't want me to touch this, but I want to touch this... mummy doesn't want me to do this but i do..." the coin drops for them that after thinking from birth that they are the centre of the universe and that everyone must want what they want, that they have a separate mind to everyone else. it's a big deal. so they test it ( and you in the process) time and time again.
If you set up a struggle after a while they do just become defiant . The book does help you gain perspective on the fact that they are not trying to drive you crazy, rather exploring a very complex world with lots of rules they don't yet understand and making the massive realisation that they have separate needs and wants to others. have you ever seen a toddler watch for their parents reaction as they inch toward something they know they are not allowed to touch? This book says they are wanting to define constantly exactly how different they are to the people around them. It helps them establish their sense of self.
Secondly and more importantly, toddlers don't like to hear the word "no" . we get lazy and over use it. Some preschools in Candada now ( not sure about US) have a specific language where they deliberately avoid negatives in their speech.
eg at dinner time if my son is playing with his food,
" we play with our toys, we eat our food" -no need to throw in a gratuitous don't in there... i.e. we don't play with our food...
he used to put his shoes in his mouth all the time,
"shoes are for our feet, it's food that we eat" apologies for the cheesy rhyme .
It takes effort , a new way of thinking and phrasing things and it may be very hard to get everyone else on board but I have found it makes a big difference. Coupled with the classic toddler exercise for parents of always reinforcing their good behaviours to eradicate bad ones, you can respectfully tell them you are going to ignore them until they stop whining or doing whatever it is you don't like . The SECOND they stop, smile, stroke their cheek or at least give them your full attention. They will start to learn pretty quickly . sometimes they need a refresher so to speak.
You will have the additional benefit of no reallly meaning no when you most need it to in an emergency. The word retains it's value and weight.