Plus--Ann Kursinski exercises, which I also call "blue in the face exercises."
I usually do this with show jumping but you can adapt it to XC!
1. Two fences at least 7 strides apart. Trot the first one, halt, back, trot the second one, halt, back.
2. Trot the first fence, walk, trot the second fence, halt.
3. Trot the first fence, trot the second fence, halt if needed.
4. Canter the first fence, trot the second one.
Throw in halts or backing where needed. This works with many, many horses.