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.
Loading or Unloading Things Defense
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
On major legal holidays, "No Stopping", "No Standing", and "No Parking" signs are suspended, except when they say "ANYTIME", does not indicate a time, or where those rules are in effect seven days a week. The major holidays are: New Year's Day, Memorial Day, Independence Day (Observed), Independence Day, Labor Day, Thanksgiving Day, and Christmas Day. If you get a ticket on these days, you will likely have your ticket dismissed.
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.
Additional resources provided by the author
The simplest way to fight your parking ticket is to request a hearing by mail. Sign and check the “Not Guilty” box on your ticket. Write a letter explaining why you should be found not guilty of the violation based on one of the above defenses. Remember to enclose any evidence that you have. [Evidence can include, but is not limited to: testimony, photographs, repair bills, police reports, Department of Motor Vehicles or insurance company reports, and towing bills.] Mail your request to: NYC Department of Finance, Hearings By Mail Unit, P.O. Box 29021, Cadman Plaza Station, Brooklyn, NY 11202-9021. Always remember to keep a copy of everything that you send to the Department of Finance for your own records.