How do I stop my toddler from constantly saying "no!" and "mine!"?
Another interesting thing about the word "no" is that it is very powerful. In fact, in any power struggle, the person in a position to say "no" is in the more powerful position. One way to overcome this handicap is to structure your requests as a "when/then" choice. For example, you've asked your child to pick up her toys and she says "no." Think about something that she likes doing that would normally occur sometime later—her afternoon snack, being read to, playing outside, etc. Then connect the two by saying, "When you have picked up your toys, then you may have your snack." If she complains, stay firm and calm and keep repeating the when/then scenario.
When using when/then choices, be careful not to turn it into a reward by bribing her with something special. Instead, simply pick two things that already occur and order them so that what she doesn't want to do must happen before what she does want to happen. It's a sort of "work before play" philosophy that will have many benefits for her down the road as well.