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 hydrant violations, the enforcement officer must describe how many feet away your vehicle was from the hydrant [even if it was 0 feet]. Failure to do so results in a defective ticket. NOTE: In NY, you must always park at least 15 feet away from hydrants... roughly the equivalent of 3 sidewalk squares.
Daytime Standing Defense
A seldom known rule is that you are allowed to park at a hydrant so long as  it's during the daytime;  a licensed driver sits behind the wheel of the vehicle; and  the driver is ready, willing and able to move the vehicle upon request. Have an letter written up by the driver explaining the circumstances and the ticket should be dismissed. WARNING: The daytime standing defense does not work at nighttime... because then it wouldn't be daytime. The definition of "daytime" is the time when the sun it up til it comes down. [I know it's obvious, but I just wanted to be clear.]
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...]. WARNING: You must be stopped legally at the hydrant in the beginning to use this defense [as in driving along the curb as a moving traffic lane or directed by live police officers].
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. WARNING: You must be stopped legally at the hydrant in the beginning to use this defense [as in driving along the curb as a moving traffic lane or directed by live police officers]. Some judges even say it's better to leave your disabled vehicle double parked and obstructing traffic instead of pushing it to block a fire hydrant.
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.
Broken Hydrant Defense - IS NOT A DEFENSE
Though some hydrants may look like they are broken beyond repair, its condition is still not a defense should you park your vehicle in front of them. In an emergency, the fire department can always attempt to repair these hydrants for use.
Invisible Hydrant Defense - IS ALSO NOT A DEFENSE
Surprisingly, EVEN AN INVISIBLE HYDRANT IS NOT A VALID DEFENSE. Due to the policies set forth by NYC's Department of Finance, parking enforcement judges are likely to hold you guilty of hydrant violations even when they are concealed from vision [like when they're buried under snow, garbage, or covered by a plastic traffic cone]. Though substantially, the Parking Bureau's justification is the same as in the broken hydrant situation, personally I disagree with these judgments. It's one thing to have notice of a hydrant ,but it's an entirely different case when the driver has no idea that a hydrant even exists.