Glue two little blocks--one inch square--oops, 2.5 cm square--just above the lamp in front and just above the strap in back just to keep it from riding up. Could be as simple as a douple layer of corrugated cardboard. Check it every now and then for security and reglue as needed. I am a big fan of hot melt glue for something like this.

Or add a little sticky-back Velcro to the helmet and lamp. Cushy side on the lamp in case you want to wear it on your forehead some time.