Spare the rod, Spoil the child!
This saying means that if you're too easy on your kids they will get spoiled. (The rod means something you hit them with.)
OK, even if you don't beat your kid with a rod, the concept of punishment is important.
By definition, Punishment means to impose something unwanted as a consequence for an action. Spanking is a thing of the past. Now we take away privileges, or give a time out.
There are all sorts of technical terms - positive reinforcement, negative reinforcement, positive punishishment, negative punishment, and so on. Don't worry about all of the psychobabble.
It all comes down to these four choices.
Giving them something that they want. - If you do your homework, you get a sticker.
Taking away something that they want. - If you don't do your homework, you lose your phone.
Giving something they don't want. - If you hit your sister, you get a time out.
Taking away something they don't want. - If you do your homework, you don't have to do the dishes.
As it turns out, number 1 just works the best. It's just a bonus that it happens to be the nicest.
Number 3 works the second best, Number 2 works the third best, and number 4 works the least.
Make sure the punishment fits the crime. If you are too harsh, you create an obstacle to attachment. If you are too lax, then you create an obstacle to growth and learning. If you reward bad behavior, even by accident, you are increasing that behavior. (See: Too mean Too nice.)
"But isn't giving a kid a sticker for doing his homework bribery?"
Great question. Technically no. (See: "Bribery")
Summary
Whether you give your kid a spider man sticker a new car, a hug, or just verbal praise, getting a reward for positive behavior is the most effective motivator. But there still is a need for punishment. Try not to make the consequence too far in future. I've never seen a kid behave himself in the morning to avoid losing recess at the end of the day. When you do threaten a punishment, make sure you follow through, or they won't take you seriously the next time.