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.
"T" Intersection Defense
A new law was enacted that allows parking at those "T" Intersections where the adjacent (major) street is not marked with a crosswalk and not controlled by all-way stop signs or traffic signals. You may park in these spaces even if there is a curb cut at that location. Be careful here, though DOT has confirmed that the law has been changed, it still has not been updated in the actual rule book. See below for Fox News' investigation into the pedestrian ramp law scandal.
Invisible Ramp Defense
If you have no notice of a pedestrian ramp, then you should be able to get off the ticket. It won't be easy to prove this defense where the pedestrian ramp overlaps a crosswalk, but it's easier for those ramps in that exist in the middle of a city block. Just explain why the ramp could not be seen. Was it covered up by snow or garbage? Write a letter and take some pictures of the location identified on the ticket and a close up of the covered ramp.
Loading or Dropping Passengers Defense
A pedestrian ramp 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.
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.
Partial Blockage - IS NOT A DEFENSE
Partially intruding upon the pedestrian ramp is not a defense to the this violation. Even if it's just the nose of your car butting into the pedestrian ramp, that would be enough to find you guilty.