Defective Ticket Defense
Defective tickets will be dismissed even if you are caught dead to rights. A ticket is defective when a required element under the law is missing, misdescribed, or illegible. When a ticket is disputed, the judge should review the ticket for these defects and dismiss them automatically, but it's always helpful to point them out. Some necessary elements are  License Plate Number;  Vehicle Description;  Accurate and Detailed Description of the Location (must say whether its in front of the address given or across the street from it);  Name of the Enforcement Officer; and  the Signature of the Officer.
Failure to Describe Violation Defense
Similar to the defective ticket defense, every ticket should provide an adequate description of the alleged violation. When it comes to muni-meter parking violations, the enforcement officer must describe  the muni-meter number;  time first observed (if it's overtime parking);  time allowed to park;  whether the muni-meter was operational;  whether there are conflicts on the ticket (most common is that the officer writes "broken muni-meter" but checks "operational" on the ticket); and  the days and hours which muni-meter parking regulations is enforced. Failure to do so results in a defective ticket.
5 Minute Grace Period Defense
As of November 2009, there is a five minute grace period for the time when muni-meter parking rules come into effect. That means if you are ticketed within 5 minutes of when your time expires, you have a valid defense. Say so in a letter to the judge and your ticket should be dismissed.
Broken Muni-Meter Defense
When you park at a space with a broken muni-meter, you can stay there for the maximum amount of time allowable by the posted signage. However, if you still get a ticket, you should immediately call 311 and report the broken muni-meter. The ticket should have a Broken Meter option for you to check on the back. Plead "Not Guilty", mark the check box for a Broken Meter, and send it to the appropriate address for Broken Meter claims. Even if they find that the muni-meter was operational, you could still plead your case to the judge. Wait for the Meter Unit to send you back the results of your Broken Meter claim, and then write a letter to dispute your ticket saying that on the day of the violation, the muni-meter was in fact broken. The judge should look at the repair history of your particular muni-meter and if there was a repair made close to the time of your violation, your ticket may be dismissed.
Loading or Unloading Things Defense
Essentially, muni-meter parking violations are "no parking" violations. As such, according to NY's vehicle and traffic law "no parking" means "no person shall park a vehicle, whether occupied or not, but may stop or stand temporarily for the purpose of and while actually engaged in loading or unloading merchandise or passengers." Therefore, if you are loading or unloading objects or passengers, you have not violated any "no parking" rules. Writing a letter explaining this should get you off the ticket. It would be even better if your passenger also writes a letter supporting you. WARNING: When unloading objects from your vehicle, remember that the defense is only valid if you unload it to the curb. If you say you were unloading or loading objects from a building, the defense may not work.
Rules Were Not In Effect Defense
Parking muni-meter regulations are suspended on the 34 legal and religious holidays. If you were ticketed on these days, you have a valid defense. The holidays are: New Year's Day, Martin Luther King Jr.'s Birthday, Asian Lunar New Year, Lincoln's Birthday, Washington's Birthday (Pres. Day), Ash Wednesday, Purim, Holy Thursday, Good Friday, Passover (1st/2nd Days), Passover (7th/8th Days), Holy Thursday (Orthodox), Good Friday (Orthodox), Solemnity of the Ascension, Memorial Day, Shavuot (2 Days), Independence Day (Observed), Independence Day, Feast of the Assumption, Labor Day, Rosh Hashanah, Idul-Fitr, Yom Kippur, Succoth (2 Days), Shemini Atzereth, Simchas Torah, Columbus Day, Diwali, All Saints Day, Election Day, Veterans Day, Thanksgiving Day, Idul-Adha, Immaculate Conception, and Christmas Day. NOTE: Sometimes due to sudden emergencies or events the City would suspend alternate-side parking regulations. Check the NYCDOT website below to see whether it applies to you.
Problems with Signs Defense
Proper signs must exist, be non-conflicting, legible, and reflect the violation alleged. Once you get your parking ticket, immediately look at the posted signage along the entire length of the block. First, see if the signs are properly placed and visible. Illegible and missing signs could give you a valid excuse. Next, read the signs. Look for whether the sign reflects the violation on your ticket. When the violation doesn't reflect the posted signs or when they conflict with other signs on the same block, then you would have a good chance of beating your ticket. To prove this defense, you have to take overlapping pictures of the entire block [from corner to corner], including the close ups of each sign and the location described in the ticket.
Can't Move Defense
You shouldn't be held guilty of any parking violations when its necessary to avoid conflict with other traffic or to comply with the directions of a police officer or traffic-control sign or signal. Therefore, when there are cars, people, or objects blocking your path and would not allow you to proceed in a safe manner, you cannot be held to have violated any parking rules. Furthermore, the verbal and physical directions of a live police officer at the scene trumps any traffic sign. Therefore, when a police officer tells you to pull over or not to move your vehicle, you have a valid defense against any parking violation. The same goes for obeying other traffic signals [like red lights, stop signs, safety cones, stop signs on school buses, etc...].
Sudden Medical Emergency Defense
If you stopped your car due to a sudden unexpected medical emergency, then you may be excused for your parking violation. To prove this defense, you must submit a copy of your medical records describing your emergency situation [ER admission records, doctor's notes, etc...]. If the medical emergency involves a child, you can have the school write a letter that the parent or caretaker was called to the school for a medical emergency involving the student. WARNING: Scheduled doctor's appointments are not considered sudden unexpected medical emergencies.
Disabled Vehicle Defense
If you received a parking violation because your vehicle was disabled, you may be able to beat the ticket. However you should be aware that according to NYC Traffic Rules, 4-08(a)8, "a vehicle that becomes disabled must be pushed to the side of the road so that it obstructs traffic as little as possible, and must be removed expeditiously." Therefore, you need to prove that  your vehicle became unexpectedly disabled;  that your vehicle was quickly removed from the location, and  explain the nature of the repairs made to your vehicle. WARNING: NYC doesn't consider accidentally locking your keys in the vehicle and running out of gas as valid excuses.
Stolen Vehicle and/or License Plate Defense
If your vehicle or license plate was stolen and the perpetrator incurred parking tickets on your behalf, you have a valid defense. To prove this defense submit a copy of the police report and explain how you promptly notified the appropriate authorities to cancel your plates. If it's a stolen vehicle, remember to include a statement from your insurance company saying that the vehicle was deleted from your insurance policy.
Sold Vehicle Defense
Similar to the stolen vehicle defense, if you sold your car and the buyer incurred parking tickets on your behalf, then you have a valid excuse. To prove this defense, submit  a copy of any documentation of the sale of your vehicle;  a statement from your insurance company saying that the vehicle was deleted from your insurance policy; and  NYS DMV receipt for plates or transfer of plates.
Authorized Vehicle Defense
If you possess a permit that allows you to "violate" the charged violation, then you have a valid defense. To prove this defense, submit a copy of the permit (front and back) and an explanation of the circumstances that caused the violation to be issued. WARNING: Handicap permits and plates ARE NOT VALID ON PUBLIC STREETS unless otherwise indicated.
Feeding the Muni-Meter - IS NOT A DEFENSE
You are only allowed to park at a given spot for the maximum time allowed by the posted signs. If it says two hour muni-meter parking, you have to move your car after two hours to another space (even if it's on the same block). Feeding the muni-meter and getting another muni-meter receipt does not save your from an "overtime" muni-meter parking violation. NOTE: be sure to look for time first observed on the ticket for a defective ticket defense though.
Getting Change for the Meter - IS NOT A DEFENSE
Running into a store to get quarters is not a defense to parking at an expired muni-meter. The only possible valid excuse is if you just parked at a muni-meter spot and you were given a ticket before you were able to get your muni-meter receipt. Showing that you got the muni-meter receipt at or near the time of ticketing would help. Also mentioning that the muni-meter is far away would help. Take pictures from corner to corner to demonstrate this point.
Able Driver Behind the Wheel - IS NOT A DEFENSE
Having an able driver behind the wheel is not enough to get you off the a meter parking violation. Using this excuse by itself without more will not be enough to get you off this violation.