If you are trading your car in on a new vehicle there are some things you can do to be sure that you get the most money possible for it. As a former used car manager for many years I can tell you that I have had people bring cars in that I would not even sit in and if your car is that filthy you can bet that you aren't going to get anything for it. There are other things that matter as well.

If you have any repair and maintenance records be sure to bring them along. If you just installed a new battery, starter, alternator, new tires, etc. it can add value to the vehicle.
If you have had any paint work on your car be sure to have the records from that as well. It is difficult to tell how bad a car was hit if the frame wasn't damaged. I ALWAYS looked for paint work and if I found it I assumed the car had been in an accident and deducted accordingly.

Remember what I said earlier about a dirty car? I am not kidding. Do you think a guy wearing $150 slacks and an $80 white shirt wants to get in a vehicle that is so filthy that he knows it is going to screw up his clothes? Of course not. Clean your car up before you even think about trading it in. Clean out the trunk, clean under the seats, vacuum it out.
If you really want to get top dollar for your car I suggest you spend a $100 and have it professionally detailed. A professional detailer can make a car that is a total dog look like a million bucks in a few hours. It will be well worth the expense I promise.

Another thing that can be an influence believe it or not is the smell of your car. If you have had mildew, had spilled milk, anything like that that would cause an odor be sure that the detailer knows about that as well. There are chemicals that can help get rid of those odors. One trick we used a lot was to take a small cup of bleach and just leave it in the car on the center console area overnight with the windows up. Usually this would clear up just about any smell as the chlorine killed the germs.

Do these things and be sure to know what your car is worth by shopping it around and looking at Kelleybluebook.com and you will be sure to get more for your car.