When you clamp the headstock, it needs to be tight, but not over-tight as you don't want to squeeze all the glue out of the joint under pressure (just the excess in order to bring the two surfaces together in close proximity). The hand-operated quick-grip clamps are probably best, as you can get a reasonable clamping force, but not as much as with a standard G-clamp. Of course a standard G-clamp is fine, but you've got to resist the temptation to just tighten it that bit extra.