Spirals - Add or start with a spiral to add to your Zentangle work.
See Michele Beauchamp's A Zentangle Spiral Guide
Shading - Add shading to different parts of your Zentangle.
See Marizaan van Beek's technique on adding shading at TanglePatterns.com
Also see Sandy Steen Bartholomew's information at TanglePatterns.com
Translucence - Make object appear semi-transparent.
See Sandra Strait's details on TanglePatterns.com.
She gives tips for both color and black and white.
Stippling - Use stippling to create shading or texture to your Zentangle.
See Genevieve Crabe's information at TanglePatterns.com.
Draw a Ribbon - Adding a ribbon to your Zentangle can be a bit tricky to get placed correctly.
See Sandra Strait's article at TanglePatterns.com.