There is much good advice here, and I would like to emphasize a few things.
Alan mentioned three 5' ground rods and Brian's illustration showed three 6' rods. I would suggest using 8' rods ... I believe that's a standard length. It's amazing what an extra 2 or 3 feet can do for conductivity.
Alan you didn't specifically say what configuration you plan to use for the wiring of the fence. The illustration shows an alternating hot/ground pattern, and I think this is very important. Doing this means that the fence doesn't require the animal to have contact with the earth to get a zap. The pattern shown uses 3 wires, but I've also used 5 alternating wires. Just make sure the wire closest to the earth is hot, then alternate from there up. This system doesn't rely on the conductivity of the earth to deliver the zap.
Finally, get a fence charger with a high power rating (joules). At 1/2 acre, the length of your fence is about 1/8 mile, so even the smallest charger would work. But for only a little extra money, the stronger charger will provide a lot of extra advantage.