Especially when forming a new behavior/habit, it can’t be stressed enough the importance of specificity. You’re frustrated that a child won’t “clean their room”? Try “pick up all your clothes and put them in the laundry hamper” instead.
“Clean” most likely doesn’t mean the same to your child as it does to you. Until it does, you have to be very specific and nagging doesn’t work!

​A related tip is understanding how important STARTING a behavior is and how sometimes a child doesn’t understand (even though you think they should or do) how to begin–so assist them, or at least offer assistance.
E.g., if they’re not responding to your request to “do their homework” try instead “get out a pencil and your assignment and lets look at it together”–research shows it is far more likely to get the result you ultimately desire. Relatedly, if they don’t do their homework, and need to do an hour. Start with 10 minutes and build up.
Research shows that children are no different than adults about the trusim that even if an adult “knows” something (e.g., eat healthy, exercise regularly, it doesn’t automatically translate into behavior. Hands need to be held until a true HABIT is FORMED). See the PDF below: 

One of the simplest ways to gain greater compliance in children is to offer choices. A command to bundle up before going outside is far less effective than saying “Before we go outside, do you want to put on your black sweater, or this blue coat?”

Saying “Please,” simple as it sounds (in a gentle tone) also statistically affects compliance.

Another related tip is turning in the task into a (playful) challenge e.g., “I bet you can’t put your snow things on in under two minutes. You’d have to be a grownup to do that!”

Punishment is a very tempting standby technique of parents, and has the added (addictive) benefit of very often stopping an unwanted behavior in the moment . BUT consider:
#1 Research shows that the effects of punishments like yelling and reprimanding or physical punishment doesn’t last beyond that brief moment (it stops the behavior almost completely because of interruption or shock rather than learning or teaching replacement behavior)
#2 Punishment often REINFORCES the negative behavior because it gives a lot of attention to the child (a form of reward, for some kids, their MOST DESIRED reward)
#3 Not only is there potential emotional problems associated with punishment, there is very often a toxic cycle of punishment: parents think that to get the same effect you have to increase the volume, intensity, or duration of the punishment (which science tells us is not effective) as well as leading to parents using illogical threats or inappropriate physical force that quickly escalates.
#4 Any physical punishment tends to MODEL that same behavior in the child, teaching them that an acceptable way to deal with problems with their peers or siblings is through hitting or leveraging violence. Further, using violent punishments greatly increases the chances that the child will respond with violence in the moment to the parents as well (subtly reinforcing that that is normal or acceptable, and linked by science to MORE aggression, and more opposition in the long and short term). 

Now, DON’T think that I’m saying you should not punish at all–it has its place–but you must reprogram your thinking to ensure that punishment is a small part of your overall approach: the focus must be on encouraging and reinforcing positive behaviors. See Child a Brat? Use Rewards, not Punishments

A related misconception is that a child engaging in bad behavior is a “teaching moment”–it’s really not. Just like it’s ineffective to teach someone to swim when they’re already drowning, the best teaching moments is repeated practice of the positive/wanted behavior when everyone is calm.