This is by far the #1 thing. Humans are adaptable. If you (and partner) stay 100% committed to something kids will accept it as reality and move on. Waver for a moment and they see an opening. Just last night there was a stream of whining and fussing over dinner. After 5-10 minutes of us calmly saying no, sit down at the table, no you can't have something else, no you can't get up and go play they both just gave up and ate dinner with no complaints.
At times it can be a delicate balance between enforcing the rules that are truly important, being sensitive to their needs/capabilities, and letting go of things that don't really matter.
#2 thing is offer perceived choice, or give them a "win". Don't ask if they want to get dressed, offer them a choice between two outfits. Being stubborn about dinner: Do you want a fork instead of a spoon? Put it in a bowl? Some ketchup? Soooooo many times just giving them something to feel in-control about works wonders.