@aart I beleive, is also a good resource for this.
Many electric fences fail because of bad grounding - that you are grounding an area so small helps immensely.  I have sandy clay soils, well above the water line (hard to believe, living in FL, I know).  When it hasn't rained in a while, I could casually touch the fence, and though it pushes 2 joules, it was merely annoying.  After a rain, when the soil conducts well?  I can hear it arcing up to 1/2" or so to any available path. 
So I ran (from the earth up) hot, ground, hot, ground, hot and then added additional grounding bars essentially every 150' or so.  That way, instead of the furthest point being 1/4 mile from completing the circuit, its not more than 75'.  I no longer casually touch the fence to test it.  Even dry season, I do so with trepidation.
If you have dry rocky soil, adding another ground at the furthest corner, connected to your ground wire will help - or grounding a secondary fence to ensure a short path.  After that, its a matter of number of wires and spacing between them, such that any likely predator has to make contact with a hot wire and some source to ground at the same time.