This is a GREAT article by SmashingMagazine for techniques of UX sketching. If you notice, it is very similar to ID sketching. Start with lighter grays, then progressively define and flesh out. It’s a great reminder for those of us who are in IxD, but never really realized that you could adapt ID sketching to UX sketching.
Article:
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http://uxdesign.smashingmagazine.com/2011/12/13/messy-art-ux-sketching/](http://uxdesign.smashingmagazine.com/2011/12/13/messy-art-ux-sketching/)
They’ve even got a nice video: