When the first coat is dry...
(Try to cover all your white canvas spots on the first coat if you can.)
Cover the whole painting again.
Think about starting in a place that will keep your hands out of wet paint. (At the top for landscapes.)
Try for more accurate color
The second coat should be richer, more vibrant, easier to match than on the first coat. Sometimes a color that's off on the first coat shows through and makes a more interesting second coat.
Aim for more contrast
Odds are you'll need to push for darker darks to achieve the illusion of more volume.
Highlights and reflections
In addition to darkening the shadows, don't be shy about making your lights light. Get more detail in general.
Final touches later
You're still focused on a general (albeit more detailed) covering of the whole painting. You can come in with polishing touches later.