For blender 2.8 use draw_xray_2.X. Button is now located in:
Top bar -> 'Viewport Overlays' popup panel (object edimode popup panel).
Draw Xray addon will help you to draw retopo mesh on top of your highpoly mesh, without problems of buildin blender xray (no proper backface culling).
How to use:
grab addon from here Draw Xray
install it by searching addon by name 'Xray'
go to Properties panel -> shading - Draw Xray (blender 2.79)
After installation you will be able to see your mesh now and few more control sliders:
Object offset - will determine how much your lowpoly object will be rendered in front of other objects. Set it to 0 to get default blender behavior.
Polygon color opacity
Edge /Vertex color opacity.
Some more info on my Blog
Update 2.2 Blender 2.8 only (17.07.2019)
added option to toggle DrawXray overlay per object; located in Object tab -> 'Viewport Display' Panel
added option to toggle snapping per object (paid version only); located in Object tab-> 'Viewport Display' Panel
fixed orthographic view rendering
fixed selection not being updated in sync with mesh
fixed crashes with snapping (paid version)
added option to customize selection highlight
Update 2.3 Blender 2.8 only (23.08.2019)
fixed crashes on scripts reload, and other bugs.
by default per object snapping is now set to 'Global' . Saves few clicks: when you enable global snapping, all objects will use now global target. You can still change it per object in: 'Object Viewport' panel to: Disabled or Local (different snap target for each object)
Update 2.7 Blender 2.8 and 2.9 (19.11.2020)
addon draws more uniform edges now
more snapping options (based on shrink wrap modifier)
Simplified UI (there is no longer separate panel in object properties)
Update 2.8 for Blender 2.93 and 3.0 (01.12.2012)
Added compatibility with Blender 3.0