There are 3 basic elements to any 3d render; if any of them are sub-par, then the quality of the render will suffer...
1 - The Model. Make sure to pay attention to surface transitions and be sure to add fillets and chamfers to get edge highlights.
2 - Materials. Use realistic materials and don't make everything overly shiny. Remember that there are very few occasions that you'll want a mirror-like finish.
3 - Lighting. Lighting can add a lot of drama and energy to a render, but bad lighting can also make a render look dull and boring. Remember that realistic lighting also has indirect illumination and without it, the render will look very fake and computery (technical term).