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.
Vacant Curbside Space Defense
The definition of "double parking" is "standing or parking on the roadway side of a vehicle stopped, standing or parked at the curb." IF THERE WAS A VACANT CURBSIDE SPACE, then you have a valid defense because the charge should have been a "traffic lane" violation instead of "double parking" violation. Write a letter explaining that the ticket is defective because there was a vacant curbside space. If you can take photographs at the time of the offense or have witnesses also write you a letter, it would improve your chances of winning on this defense.
Loading or Dropping Passengers Defense
A double parking violation is actually just a "no standing" violation. As such, according to NY's vehicle and traffic law "no standing" means "no person shall stand or park a vehicle, whether occupied or not, but may stop temporarily for the purpose of and while actually engaged in receiving or discharging passengers." Therefore, if you are loading or dropping off passengers, you have not violated any "no standing" 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: Picking up or dropping off passengers DOES NOT INCLUDE INANIMATE OBJECTS. Loading or dropping off objects [not live human beings] is not a defense to a "no standing" violation, even when it comes with the person. As ridiculous as it may seem, if you pick up a person in a wheelchair, once your pick up the person you may be found guilty of "no standing" should you load the wheelchair as well.
Commerical Vehicle Working Defense
Commercial vehicles are permitted to double park when they are expeditiously making pick-ups, deliveries or service calls, so long as there are no curbside parking available within 100 feet of your vehicle. To prove this defense,  your vehicle must have a commercial type plate;  write a letter explaining that you were stopped for the purposes of expeditiously making pick-ups, deliveries or service calls; and  include a letter from your employer explaining the nature of your job and that you were working at the time of the violation. WARNING: this defense DOES NOT WORK IN MIDTOWN MANHATTAN (the area from 14th Street to 60th Street, between First Avenue and Eighth Avenue) from 7AM to 7PM daily except Sundays.
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.
My Hazard Lights Were On - IS NOT A DEFENSE
Just because your hazard lights were on does not mean that you can double park. If there is a valid emergency (like sudden unexpected medical emergencies, disabled vehicle, or your inability to move your vehicle in a safe manner due to surrounding circumstances), then use those explanations as your defense. Activation of your hazard lights do not grant you the privilege of free parking.
My Engine Was Running - IS NOT A DEFENSE
The fact that your engine was running does not mean that you were not double parked. Even having your foot on the brake and still in the "Drive" gear, does not mean that you were not parked. Using this excuse by itself will not be enough to get you off this violation.
Able Driver Behind the Wheel - IS NOT A DEFENSE
Similar to the engine running excuse, having an able driver behind the wheel is not enough to get you off the a double parking violation. Using this excuse by itself without more will not be enough to get you off this violation.
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.