Here are some things to try...

1. go to camera attributes > environment and make the background color something other then black (BTW this is one of my biggest pet peeves about posts - the default black background makes almost all renders look like crap!)

2. Add a ground plane (something as simple as a single poly plane will make a big difference). This also helps final gather.

3. For quick high quality renders I just go to the quality tab and select production fine trace and then go to the features tab and select final gather. Just use the defaults. This is assuming you are rendering in mental ray.