Keep her busy. When you feel her start to get nervous ask her to do something.
This puts that nervous energy to work.
Also if she realizes that when she stays calm its less effort than when she gets jiggy she will more than likely learn to relax.
Got a friend with a steady-eddie trail horse to ride with? It's also perfectly fine to tack her up and hand walk her on the trails, I did this with one of mine at the beginning, then alternated between riding and handwalking in the areas she was more comfortable in.
You can also teach a head-down cue, it helps calm a horse down - I like John Lyons' techniques, because he focuses on safety for you and the horse, and doesn't use a lot of force.