Have a look around and know what market you're in and which period of the year. A good clue it's time to sell is when you start to see more "Wanted" ads than people trying to sell their vehicle!
The pictures of your car have to look good, and ideally the car itself! Give it a nice compound and then wax it, shine your tires and if you have time complete the necessary repairs as if you were going on a long trip. This way your buyer will fell safer!
I had some leftover time at the end of my trip and spent 2 weeks fixing up the aesthetic and repairs needed... bottom line it sold in less than 24h and I made an interesting profit!