fences

Question
how do you stop an 18 month female boxer from jumping fences

Answer
Work on both "off/leave it" and boundary training, so that she learns not only to stay off of the fence, but also to stay within the fenced area.  Some good articles on these behaviors are here:

http://www.clickersolutions.com/articles/2001/leaveit.htm
http://www.clickersolutions.com/articles/2001b/leaveit.htm
http://www.shirleychong.com/keepers/archives/barrier.txt

(The "leave it" training is not specific to fences, but can be adapted and is a good behavior to teach in any case.)

While you're working on this training, you can run chicken wire along the top of the fence, in an upside-down "J" or "L" shape, so that she can't get over the fence if you have a moment of inattention.  This doesn't address her behavior, so is only a temporary management solution.  You can also keep her on-leash in the fenced yard while you're teaching her boundaries.

Good luck!