Sacking out is really pretty easy. One thing to expect is that the horse may freak out a little, but it is okay. The best place to do this is in a round pen. There are varios ways that people have achieved this. I like the Pat Parelli way.

Basically, with his way you start with a lunge line and you just swing it back and forth by the horse. For some horses, this is scary and they may back away. You just act like its no big deal and keep on swing. Start slowly at first and get faster. Some horses don't like you to even wiggle the rope. Keep in mind that you haven't even touched them yet. Linda Parelli, Pat Parelli's wife, has a great story of the first time she did this with her horse. She was wiggling the rope for hours. I don't think your horse is this sensitive. After they are comfortable with the swinging then you start to swing it around their legs, then over their back and over their head. Go slow and work on each section until the horse is comfortable and stand stills. You want them to stand still before you move on. Soon you will be able to swing the rope all over the place and the horse won't even flinch. Then you can start to introduce scarier things like plastic bags, blankets, feed sacks, anything you can think of. Start slow and let them sniff and snort. Then rub it all over their body. Slowness is key and you will want to watch their reaction. If they react take it away, turn your back and pretend like you are playing with it. Make no big deal out of anything. Your horse will start to wonder why you aren't paying attention to them and will come to you. Then you will slowly start over. This is something that you can do every day. You just want to get them used to everything that you can. Some horses are so sensitive and others could care less.

I had a mare that would spook herself with her own tail. A friend of mine tied a feedsack to the saddle with a can and rocks inside and lunged her until she got used to the sack. She freaked out a little and it probably looked more dramatic than it really was, but she was ten times better. We would leave the feedsack hanging by the roundpen so that she would get used to it and not spook. This method is quite a bit different than what I described, but it works too. Like I said, we used to go to barrel races and tie the horses up and watch our friends compete. The horses would stand all day and get used to all the other horses and racket and standing tied. This did wonders for them as well.