Trail riding is one of those things its tough to be prepaired for everything. It seems there is always something that will spook your horse, the best you can do is prepare yourself for anything. It does help to introduce your horse to as many things as you can such as the plastic bags etc. We use 4-wheelers, kids on bikes, balloons, lawnmowers just whatever you can think of that you might run into on the trail. Of course you can't release a covey of quail or make a herd of deer run up behind you but those are the things you have to have yourself prepared for. You may have to enlist the help of a friend/family member to help but its better to do some of these things in an arena to see how your horse will react to them. A friend of mine has built a "land mine" of obsticles at her place and we go over there and play sometimes. She has a wooden bridge, just a wood platform with panels on the sides, tarps with logs holding them down so the horse has to step over the logs and onto the tarp (a tough one), a mailbox you have to lean over to open and close (I've gotten good at getting the mail by horseback), gaits to open a close and several others I can't recall. Just use your imagination and set some obsticles up in your arena.
Sorry this is so long, hope it helps you. :)
Added: Do you have a road you can ride down so she gets used to cars? One that is not used much like a dirt road, not a major highway.
"Until one has loved an animal, part of one's soul remains unawakened..."
- Anatole France