I desinsitize my horse at the barn, arena, etc. I get jackets, plastic bags( a must. Don't know why horses are sooo scared of plastic bags) And I shake it in front of there face in little sections. Works great!
Use your voice calmly, show your horse that you understand the situation and his problem. Have a "let's take a look!" attitude, approach slowly, allow him to get a closer look, encourage him to approach even more and even touch with his nose. Pet him, praise his courage of approaching the "big bad thing over there".
When you are on the ground, try using the same approach, have maybe some treats handy. Associating the approach of scary things with treats gives results!
My horse used to be very spooky too but since I've been working with her she hasn't spooked once so what I did to fix this problem depending on the discipline like I'm a jumper so I would show my mare the jumps and walk the course to show her from the ground so she can see that I'm not afraid so there shouldn't be a reason for her to be afraid either. After a few days of doing this I found a real difference and slowly stoped doing this so I'm not stuck having to do this at a show. If you just ride around or go on trail rides and can't show your horse everything from the ground you may walk your horse up to whatever it is that he/she is scared of and let your horse know that it's okay and that nothing scary. If that doesn't work you can do little things to keep your horses mind distracted by playing with them a bit like weaving around cones or poles or practicing side passing or always changing the speed of your gaits like working trot/walk/canter or collected walk/trot/canter. Just try changing things up and sometimes theses little things can make all the difference. Hope this helps!