Deciding on the best worm control program for your farm can be a complex matter, but guidelines in this article by Robert Wroth should make it simpler.
"Sustainable worm control in sheep,"
Journal of the Department of Agriculture, Western Australia, Series 4: Vol. 37
, Article 5.
Available at: http://researchlibrary.agric.wa.gov.au/journal_agriculture4/vol37/iss2/5