The stuff people are saying here is true. Definitely lock your car doors, I've had my car broken into about 3 or 4 times. The key here is to stay in the good areas. There are some really nice areas in Memphis and there are some places that look like an atom bomb detinated over them, I'm not kidding and what that guy said about Lamar Ave. is not a joke. My tip is this...if you go downtown stay around Beale st., the mall, the Toyota Center, the ball-park, that area. Don't go to far from the tourist areas especially at night. Don't hang around in the far north and far south areas of the city, those are places I try to keep my visits even in the car to a minimum. Visit the tourist areas of downtown, also mid-town, east Memphis and Germantown, there all nice.
There are many areas of Memphis that are unsafe. Make sure you stay in a well lit area and I don't recommend going out alone at night. The main thing you'll need to be safe about is leaving things in your car when you go out at night. I've known a couple of people whose cars have been broken into. Either take with you things you don't want to lose or leave them at your hotel. I've never had any problems leaving things in the trunk of the car, but we always park in the parking garage at Peabody Place when we go downtown. They don't guarantee your things will be kept safe when you park in the parking garage, but I feel safer parking my car there than I do leaving it on the street.
I just want to make travellers aware that Memphis can be a dangerous place to get lost. Be sure to stay in lighted, well lit areas, especially downtown or in Midtown, and keep your street smarts sharp. For instance, if you are going out, and you are in a visible spot, don't put your bags or valuables in the trunk or car in open view. If you are on Beale, stay on Beale and don't venture too far away without knowing where you are going. Be on guard and have fun.