The description below applies to outside settings. Studio settings should have a different setup.
First off, in the perspective viewport I select Viewport Properties. I then select a wallpaper (preferably hi res - 1024 plus) and make sure it isn't displayed in grayscale mode. The viewport than has a colour wallpaper as a background image. Here in the Viewport Properties I often make sure the camera lens is set at 35mm (0.35).
By rotating and angling the model I than map it over the background in the desired place.
In Raytrace Environment I first create a 3 Colour Gradient using colours present in the wallpaper image (eg blue for sky, green for horizon and than dark grey for the ground).
I than check the Background Image box in the Advanced section (which opens up a new window) I select the wallpaper image again and make sure the Projection is set on Planar.
Sometimes I also create a Ground Plane under the car - the texture should be similar to the condition of the ground in the wallpaper image.
I than open the Sun and Sky Settings. In Specifiy Solor Angles I choose the "Directly" option. With the Azimuth and Altitude I create a similar lighting setup to the light quality present in the wallpaper image. I also change the Cloudiness and Place depending on the wallpaper image.
In Document Properties, I set the Render mesh quality on Smooth & slower (or a Custom option). I make sure the Antialiasing is on High 16x.
Than the model is rendered off

.
In Photoshop (I use CS) I than touch up the rendering. Often I overlay the wallpaper image back over the rendering and erase the area where the car is present. This gives a better transition with the Ground Plane. I also clone or copy some of the wallpaper image into the car bodywork, making sure it is at the right angle to make sense. For example, in the image above I copied the tree and ghost-overlayed it at an angle on the hood.
When all touching up is finished, I sometimes add a subtle Film Grain. Than I sometimes apply a Photo Filter to give a nice atmoshperic impression.
I hope this helps, but a good rendering technique is really a question of trying things out yourself and finding your own style. :wink: