Honestly I ground drive essentially with driving reins attached to each side of the bit. This teaches them how to listen to the bit without forcing a headset, which is honestly what side reins do, like others said. I never once used side reins on my Arabian mare, and just with lots of ground work, ground driving, and letting her get used to the bit, she went from head in the air, not sure what I was asking, to putting her head down, stretching towards the ground, and accepting the bit. I train dressage primarily, and no matter what the horse's age, there is a set group of stages that have to happen. First, not messing with the head, teaching them to listen to the seat, and if their head is in the air, that's just part of learning. As they learn to flex and bend, and start to become a bit more balanced, they start lowering their head, and stretching forward. We then spend a while just pushing them forward, and letting them stretch down into the bit, and lift up their back. After that, we then take up more feel of the mouth, start asking them to really move underneath themselves, and into the bit, and naturally as they balance properly, and move under themselves properly from back to front, their head naturally ends up in the "proper" place.
I rode two horses for quite a while both about 6 yrs. Old, both started by the same person, and both asked from the get go to have a head set with draw reins and side reins with no proper engaging of the hind end. The mare was ALWAYS behind the vertical, never wanted to have contact with the bit, would rear and stop her feet when she got frustrated, and had no concept of moving forward. Had that problem though she was getting better, up until the day the owner gave her away to someone because she couldn't deal with the horse's issues anymore. The gelding was the opposite, if you didn't have tight tight grip on his face, he'd just careen around the arena completely out of control, would stick his head up in the air as soon as there was any slack in the reins, and never stretched down. Not a whole lot was done with him before I got my hands on him, and he started out doing pretty good, then I moved for a year, and another person rode him for the owner, and created all these problems again. I never lunged him in side reins, and really asked him to move forward, so he was actually going pretty nicely when I left. When I got back, he needed way too much contact, had no concept of lifting his back and moving forward without taking off, and while he was definitely getting better when I moved again recently, he still rushed, and still was upset with looser rein contact. For the first few months after starting riding him again, all I worked on was walk and trot, moving off my leg nicely, and asking him to balance himself properly. It took quite a while before I would canter, and even longer still before I would canter the entire arena. Knowing the owner, he's always going to have issues, as she thinks he's just fine, and she's fine with having a death grip on the reins, but as others said, its much harder to fix a "head set" than it is to get them to properly give to the bit and place their head according to balance over time.