A few things to try in my next sketches & watercolors.

Tone the page before you start sketching. From artist's blog:
http://teresenielsen.typepad.com/the_world_of_terese_niels/2012/06/blank-slate-confidants.html

Watercolor plus pen and ink. From Pinterest

Start with wash and draw over the top.
https://creativecolor.wordpress.com/tag/elements-of-art/