Before you ride you can walk him around the arena to show that everything is ok.You could also go on trails or do excersizes using tarps,bags, or you could even make loud noises to get them used to that.Hopefully this helped!
Show him/her that there is nothing to be afraid of by leading him/her around the things he/she gets spooked at. Let is get to know it. Also doing a lot of ground work can help.
If ground excercises don't work as well Smartcalm supplement from Smartpak works wonders and it's good for your horse!
lead him to the items slowly and let him sniff it and then touch yourself with it then him