I've found the Davis Wind-Tels to be helpful in super-light, shifty air. They attach to the shrouds with a wire that corkscrews around the shroud (I taped over them to be sure the end of the wire didn't tear my main).

This is really only needed when you're out in wind so light that you shouldn't be racing in it anyway, such as is sometimes found in a distance race or on the way back in after the wind quits, etc.

A mast head indicator would be even better, but I have yet to see one for a Hobie 16 with the rake we carry.

Mike