Mike Andrew Nash has recently released a guide to designing nice-looking sci-fi models in ZBrush.
The artist shared tips on creating a space marine discussing his work called Juggernaut unit - 01. It's a detailed video tutorial with tons of tips on great character design. Basically, the tutorial will teach you how to create high-end concept designs inside ZBrush.
Nash talked about creating a design from scratch using standard tools covering the concept design and refinement process. Please note that guide doesn't cover rendering or Photoshop work.
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You can learn more and get the tutorial for $52.95 here.