the higher you can mount the light the better, to offset the tendency of the light to pull the helmet forward. other than that, make your straps tighter.nathasm said:
If your helmet is a little big, even w/ the foam fitting pads the helmet will move around a lot w/ a light on top. fit yourself to the smallest size helmet you can fit in, eliminating some of ths slop and play that you can get w/ a larger helmet.
I wear a Giro E2 and route the wires through, and out the back vents of the helmet so that the power cord is not in the way.