Few things in life can ruin a good mood faster than a big orange parking violation hanging off the side of one's car. I have been fortunate and have gotten myself out of the vast majority of tickets. Read on to find out how you can, too.
In Chicago, we have the option of sending in a written dispute of the ticket within 2 weeks of the incident. It's called contest-by-mail and I always opt for this route. A clear, well written letter with photos is much easier for you and hard to refute for them. If it doesn't work, you can always go to court and hope the police officer doesn't show up.
Writing your letter
Write a considerate, well thought out letter. State all of the facts using absolute locations, addresses and landmarks. If the ticket is about parking at the wrong time then make sure to list the time the ticket says, the time you know it was, how you knew and what you did at that time to make sure you weren't in the wrong. Make sure to always list that you clearly saw and read the signs and knew you were absolutely within legal limits and were not contradicting the sign. It's also a good idea to indicate how you believe the officer may have made the mistake.
Use clear terms and NEVER try to confuse the person reading your letter with jargon, lesser-known words, metaphors or anything that isn't clear cut and related to the case.
Remember that the person reading your letter probably doesn't like their job, probably gets yelled at for letting too many tickets slide and by default is expecting you to be wrong and looking for any reason to mark your dispute as invalid. Be nice! An angry letter will get a quick, decidedly angry response. It's better to be apologetically assertive than to come across upset and bothered by the whole ordeal. You'll get more sympathy that way.
Thank the reader! Show them appreciation for actually bothering to read your letter before stamping it as pay-anyway. This will make them feel right about their decision. We, as humans, really enjoy doing good things for those that we feel deserve it and this is no exception.
Citing local traffic laws
Be careful with this one. If the ticket is for something obscure and you know you're in the clear given a local law that perhaps the ticketing officer was not familiar with, then maybe it'd be a good idea to casually reference the law or ordinance. The main thing is that you don't want to sound arrogant or be construed as a "pocket lawyer." Only cite laws and ordinances when they aren't clear or you're sure they need to be displayed for everything to be understood.
Check your local defenses, often times they are published on a city website. For chicago, they are available here
Always take photos of your care, the area and the signs that affect the incident. Show clearly that you were not in the wrong. Add illustrations if you have to. Anything you can give that they know visually is correct will further support your case. I recommend taking digital photos, printing them all on one sheet and explaining in your letter what the photos are depicting and why they are proving your innocence.
Putting it together
Put your letter and photos all onto one page if you can to make things the easiest for the person auditing you. If that's not possible, then start with the letter and put the photos on the second sheet. Use a 3-sided fold and make everything look as neat and professional as possible. Make sure to mail it in on-time as to avoid invalidating your contest. If you've made a clear-cut case for yourself, then hopefully you won't hear about it again and your ticket will have been tossed.
A letter that has worked several times
Below is the letter that I wrote to contest numerous tickets in the exact same location over the course of about 2 years. You'll notice that I used all the techniques above in the written portion and assembled it with pictures on the first page. I also printed out full-scale photos onto 2 more pages (1 per side) and submitted those so that the signs could be clearly read. In this example, my car was parked between two permit-only signs that designated all the areas except mine as permit, meaning the spot that I was parked in is free for the public. I've given a screenshot of the HTML as well as a link to the HTML itself.
Update 1/18/2009 - I got out of another one. This time I was parked in a permit-only spot in Chicago and got a ticket for not having a permit. Apparently the officer did not notice that I had a temporary permit up on the passenger side of the car. I wrote another nice letter explaining how I could see how an officer would miss this at 3am and thanked them for their understanding. It worked.