Be smarter than your neighbor. If the subject of the yard turns into a debate, use indisputable logic.
Don't get in a fight or threaten your neighbor with physical violence. This will only make the situation worse and almost certainly will not result in a clean yard.
When a neighbor has a messy yard, it's likely to affect you in two ways. The first is it's an eyesore and can be bothersome to look at. The second is that it can lower the value of your home as well as the neighborhood. This is particularly true if you are trying to sell a house, and a potential buyer is hesitant to live next to your messy neighbor. Many approaches can resolve this, but the ones you pick will depend largely on the relationship you have with your neighbor and your neighbor's attitude.
Think the subject over, and decide approaches you can employ to get your neighbor to clean the yard. Consider if she has extenuating circumstances, such as an illness, a recently born baby or an extended period in which she was away. Remember some people are just naturally messy. Have a plan. This will help prevent you from approaching the neighbor in an overly emotional and disjointed way.
Try the carrot-over-the-stick approach first. Appeal to your neighbor's better nature, and bring up the subject of his messy yard with a smile on your face. Use a little psychology. Sometimes you can persuade people to do things without their even realizing it. Speak in a friendly and relaxed tone.
Use some commonsense arguments if the nice approach does not work. Say the mess is affecting the value of your property. This will be particularly effective if you have a for sale sign on your lawn. Bring up potential health hazards if the neighbor's yard is littered with garbage or dog poop.
Try some other approaches. Offer to help clean up the yard. Try to get other residents in the neighborhood to talk to your neighbor and join in the effort. The neighbor might be willing to listen to someone else. Offer to help pay to clean up the yard. Some cleanup services will be glad to do it for a price. Bribery might be too strong a word, but people do listen when the subject is money. If a yard is extremely messy, it's possible your neighbor is not cleaning it because it's too daunting a task. If you offer help, your neighbor might look at the job in a different way. Have your real estate agent talk to the neighbor if your house is for sale. This will really drive home the point of the value of your home.
Put your foot down by firmly telling your neighbor the situation has become intolerable. Tell your neighbor that if he does not address the mess immediately, you will go to the authorities. If that still does not work, go to your local government and make it aware of the situation. There are ordinances and zoning codes against dumping and other sanitation violations. The fact that your neighbor could be fined might finally result in some action. Contact the government again if it fails to respond to your complaint.