After a rendering is completed (assuming you at least used Option > ray trace quality > Level = 4 and Anti-aliasing = 5, with Presentation GI Density set to ‘More’ ) , when I Save my render I open it in IrfanView and do three things.
First I use the Sharpen option in IrfanView. Be careful not to over sharpen or pixels appear.
Then I use the Image > Color Correction option and reduce the Gama slightly. This darkens image and brings washed out colors out.