First, keep in mind that horses will come out of frame because, well, it's HARD to keep yourself round when going in a circle over poles (hence the muscle building thing ;) )
If you have one that stargazes while lunging, first make sure you are going s l o w. Really slow - like a jog. I always then take the lunge line and gently shake it from side to side as if saying "hey, pay a bit of attention here"...don't know why, but even with my 5 gaited (aka upheaded) saddlebred, this works in lowering his head. Keep the momentum slow, slow, slow until your horse is very comfy doing it. I always know when my guy is really into it...he kinda sashe's (sp?) really moving his butt in an up and down fashion as he moves.
The Pessoa lunging device is a great mechanical way of getting your horse to round out. It can be pretty easily immitated in parts. http://www.saddleupinternational.co.uk/images/b817.jpg
The big arab barn I worked at would take a surcingle and two lead ropes (or split reins) the split reins were tied to the sircingle (about mid way up), run in between the horses front legs, then hooked to the bit. As the horse progressed in his training, the reins were tied shorter and shorter. (word of caution, this does not get them "on" the bit, more like behind it - and many would rear at the unfamiliar pressure...so be careful)
gosh, that turned out long... good luck!