To create a good impression on a potential buyer you need to prepare your car so it's looking its best when the buyer arrives. Consider having it detailed by a professional to save time.
If you opt to clean it yourself make sure you clean in all the hard to get at areas, like door openings, boot openings, and the engine bay. Never leave anything lying around the interior or boot making them look untidy in the eyes of the buyer. Loose items can rattle and bang around and create a poor impression on a test drive.
If you're a smoker clean the ashtray and make sure the interior doesn't smell of cigarette smoke. There's no better way of putting a buyer off than to show them a car that smells of tobacco. If it does smell get a deodoriser and leave it in the car for a week or two before advertising it in the hope the smell will go away. Don't leave the deodoriser in the car when showing it to a potential buyer. Wash and polish the exterior including the wheels, windows and all chrome or stainless trim. Clean up any residue of polish after you're finished to avoid giving the impression you've just cleaned the car up to sell it.
Consider having the engine oil changed a week or two before selling it so it looks nice and clean if the buyer pulls the dipstick. Make sure the windscreen washer reservoir and radiator are topped up to the marks so it looks like you have cared about your car. Take the time to inflate the tyres to the correct pressure. There's nothing worse than going on a test drive and being shaken and shimmied because the owner didn't set the tyre pressures.
Have the car's service record on hand to show the potential buyer that you have had the car serviced regularly. It's another way of demonstrating you've looked after it. Also have the car's title on hand to show that you are the owner of the car you're selling. Consider investing in a vehicle history report, this will set you back about $30.