It's the nature of cars to lose value, so step one is accepting that (unless you have something ultra-rare). But what comes next?
Let's assume you've decided to sell it. What's next? You're going to need to advertise. Expect to make a little bit of an investment here, but it will be worthwhile. You have a lot of options here. You can go electronic, and target a much bigger audience, or stick with more old fashioned methods and deal locally.
The Internet is going to draw the most attention to your used ride. There are plenty of places online that will guide you through the process of describing your vehicle's condition and posting photos. Take a look at Cars.com or Autotrader - these are pretty popular sites and should generate some interest in your ad. Doing a search for "sell used car" will provide you with tons more options. Another good resource is eBay Motors. You'll need to set up a seller account here, but eBay has a pretty straightforward guide to creating an auction. Just make sure you understand all of the available selling options (auction, buy it now, best offer, etc). Pick the one you feel the most comfortable with. If you're already an eBay guru, this is probably the best option for you. Just remember that this is less localized, and you may get interest from buyers in other states. Don't offer to deliver a car if you don't have the means to do it.
If you want to keep things simple and stay local, post an ad in a few newspapers. A newspaper ad will get you some good publicity, but limit your reach to local buyers. This might also give you more opportunities to show off the car in person. You can find printed auto trader magazines at news stands and grocery stores. These are still regional, but tend to cover larger areas than a newspaper would.
If you have access to property in a busy area, go ahead and park the car out there with a sign and phone number. If you can grab the eye of a few drivers passing by on a major road highway you'll have a good chance of getting some phone calls.
Once you have chosen how and where to advertise, you must create good, accurate content for the ad. Pictures are the best way to grab the attention of car shoppers. Use pictures to highlight the condition of the car, and any options that might not be standard. Your photos should be a good substitute for a live showing. If you're doing this online, you should have plenty of space available to provide all the details of your vehicle. Don't leave anything out. If you chose to use a newspaper or other printed publication, the amount of text you can use will be limited. In this case, only mention the best, most attractive features. Don't bother with the obvious - you want to use words that will stand out from the ad next to it. What you write here has to induce a phone call. That's when you can confirm all the other, less exciting details.
Know your vehicle. You must be familiar enough with the car you are selling to make shoppers comfortable with what they may be buying. If you don't know what kind of engine it has, some people will be turned away. Know its history. A smart buyer will ask about maintenance records and any major repairs. If you don't have proof of information like this, the value of your car decreases. Do some research before selling if you need to. Use your VIN to find records of work done to the vehicle - especially if you are not the original owner.
Be honest. Don't exaggerate or lie about your car's age, mileage, condition, or features to attract attention to your sale. Don't leave out important details either. If the sunroof leaks, mention it. If one of the power windows doesn't work, tell the buyer. You can avoid some nasty disputes later on by being upfront from the very beginning.
Know the car's value. Supply and demand for used vehicles can change over time, especially in an unstable economy where new car sales are struggling across the board. You may be wrong about the market value of your car. If you ask too much, it won't sell. If you ask too little, you could be missing out. You can research your car's Kelley Blue Book value, based on its age, mileage, and condition. This is a good place to start, since buyers will be looking here too.
Finally, be patient and flexible. If the first offer isn't what you want, refuse it. If you don't get a call in the first week, that's ok. If it's been a month, consider updating the description in your ad in order to draw more attention to it. You may eventually have to adjust your price. Your geographic location can have an impact on the car's value too.
If you're willing to do some research, take some pictures and wait patiently for buyers to respond, you may get far more than the trade-in value for your used car.