First, I just reread your post and I think you know that probably the best thing for the horse would be to see if you can't just turn her loose in the arena at least a few times a week.

I've started anticipating her spooks. I can feel myself tensing my back and waiting for her to react to something... she feels me tense, and gets tense herself; then she spooks more and more.

A couple of other suggestions.
* Breathe. Regular, even, calm breaths.
* As soon as you feel yourself starting to tense, immediately do something else - something easy. Maybe have some cones out in the arena and walk figure 8's or weave around them. Simple, basic exercises that both of you know how to do well and enjoy.
* If you're mounted, get off (calmly). Do something. (I would not run away from the spook inducing object, though ) If you act like you are totally unconcerned about whatever spooky object is there, she will pay it less attention.
* Don't focus on the spooky thing unless you can do so totally calmly. If you can't focus on it calmly, focus on something else entirely. I like to point out potentially scary things to my horses - many times I will see something that I think may bother the horse and I'll point it out, I'll just talk about it calmly and maybe mention that it really isn't a horse-eating monster and ramble on and on. Most times they figure that since you see it and aren't scared of it, it must not be scary. (This goes right back to the talking or singing)
* Bomb-proofing work. (I call it play) I find the more weird, wacky and scary stuff I do to my horses, the less upset they get and the faster they calm down.

What hollypossum wrote is good, too. Take her mind off of the scary objects and give her something else to focus on.