First/Then is a very effective technique, which involves the application of positive reinforcement. Basically, this means, if you do this thing first, then you will receive the thing that you want. (the reinforcer).
Examples of this include:
First eat your broccoli and carrots then there is ice-cream.
First brush your teeth then we will play with dolly.
First leave the car, then when we get inside we will play with your Thomas the Tank.
The key to First/Then is to make sure that the Then is an activity or thing that your toddler really is motivated to engage with. If this strategy does not work for you it’s because the Then part is not what they are motivated by.