Just to add to things that can speed up things, remove or hide things from the scene you don't need. Especially the standard environment (Background prop, if I remember correct) increases render times when its on, as its basically a prop around your scene that the rays will interact and bounce off, so hiding that if you don't need it will improve render time as well.

Another way you can test your scene is to render it in progressive and after a while you will be able to see areas that are more grainy than others, So you can use those as test areas using the Render area functionality and just adjust the settings until you are satisfied that these areas look as you want them. That way you will get a good idea of what settings you need.

Did a test in another post with the min and max diffuse bounces and from what we figured out in that thread was that by lowering max diffuse bounce to 1, you will loose some quality in the bounces, but it cant improve render speed quite a lot, so you can experiment with that as well and see if that works for you.

Yet another thing you can do, is to render the image slightly larger than what you need, so say you aim for 800x600 then you render it as 1000x750 instead and then scale it down in photoshop, that can reduce noise as well as pixels starts to "blur" together. Obviously you shouldn't go nuts here rendering 6000x4500 if you only need 800x600. But even though you render a larger image, you might be able to use fewer samples and therefore increase render speed without loosing to much quality.

Last if you only need one image for the cover, it might be worth as others have mentioned to simply spend the time needed to let it render in good quality. :)