I have had reasonable success with adding eight 500mg capsules of Frontier garlic extract to two gal of water. This is the mix that my neighbor suggested and I havent seen a worm/segment yet. I do sand shake tests every month or so just to keep an eye out and play it safe.
A few of the girls dont seem to like it but will eat a clove (makes no sense I know). Im told its the same effect, but Im not sure. I have a friend that swore by food grade DE for parasite cleansing. She would put a tablespoon over horse grain for each 5+lb bird.
If you can find it, Blewstons organic para-sweep is pretty good. Its mostly DE and pumpkin mash and gets pretty pricy (around $50 for 10 oz), but the girls will kill me for it! Ive only found it in Canada (PEI) but Ive had it shipped to the states a few times.