...
I'm guessing your horse is spookey, so the best thing to do would be to spook-proof her in an environment she feels safe in.
What you could do is find something that spooks her (a plastic bag maybe) and get somebody to hold her on your yard. (Don't tie her up, because if she reacts badly, could be a bit of a disaster..)
If you try and bomb proof her in an environment she feels she is safe in, then she is less likely to spook at something.
Back tot he plastic bag example, what you could do id stand about 10 meters away from her and rustle it, look carefully at what her reaction to it is. Once you think she is comfortable with it, you could stand a bit closer to her and rustle the plastic bag.
Another thing I've tried that seems to work is when you go on a hack, don't ride. Have her tacked up, but lead her. If she shys at something or if there is something that she is likely to spook at, try and get her to go over to it. Don't drag her to it, but talk to her and gently and slowly get her to go over to it and sniff it. By doing this, you are showing her that it's not dangerous.
If she is particularly strong, maybe get another person to come out with you with an extra lead rope, so if she starts going a bit silly, they can clip onto her too so you've got a bit more controll
I tried this with my three year old welshie, and it seems to have worked.