I had similar complaint from one of the customers in Russia. My suggestion was to wear the device over the sleeve or something similar to what Ket has suggested - which has been accepted so...

We've seen this previously on the old wearables. A simple solution is for the users to use a Tubigrip (athletic support see here for examples http://www.chemistdirect.co.uk/tubigrip_v_455.html) or something similar, fairly low cost (~ £1 to £2.50 in the UK), come in all different sizes, washable and comfortable. I've also seen one customer where the users used socks with the end cut off!! Bit odd but it seemed to do the job. IJ

Try putting a wrist band like http://x.couver.us/extra-long-wristbands-pro-31/ under the WT wrist mount.

Great find Ken!  Thanks for sharing. I'm currently working with Custom Products and the WT4090 Product Manager (Juliet Chon) on a similar arm sleeve made of a material that reduces slipping when sweaty, is comfortable, and wicks sweat away from the arm.  It is also an inexpensive personal item. It does not exist yet, so please let Juliet or myself know if anyone is interested in this.