How loose is the headset supposed to be?

Hey guys! I've had my Vive set up for about 3 hours now, and I had a question about the headset itself. I'm having a difficult time getting it to feel snug--I've fiddled with it for quite a while and can never seem to get to feel like it's solidly on my face.

It sort of wiggles around when I move my head, even when I have the straps as tight as they can go (ouch)

I thought maybe it was because I have thick, longish hair that's sort of "lubing" the straps, but I really have no idea.

So. How much wiggle is it supposed to have?