One thing I learned recently is that it is best not to treat the whole herd at once for worms, as that is the quickest way to have a problem with resistance. The idea is if you worm them all at once, only the strongest/most resistant worms live on to reproduce with each other, making them harder and harder to kill off. If you leave a few animals untreated(Maybe do them 3-5 days later) the "smart" worms that survived the first round of worming breed back to the "stupid" ones in the untreated animals.
Looking over my notes from the same talk (by AASRP vets at the ADGA convention) where I learned about this, it was recommended to withhold feed for 12 hours prior to giving ivermectin to increase its efficacy and that giving it orally was most effective.
They recommended using a class of dewormer for a specific job until it fails, as switching between classes can promote multi-drug resistance. An effective wormer should improve the goat by at least 1 FAMACHA score in 7-10 days after treatment.
The talk was given in North Carolina, where many many wormers have become ineffective. Lots of people there had to use additional measures to keep worm loads down, such as making sure all feed is kept off the ground, keeping goats' feet out of feeders, etc.
Disclaimer: The weather is so cold and dry where I live that we do not have significant worm problems/dewormer resistance. I do not have personal experience to back up the info I was given, and maybe others can weigh in. I just wanted to share, as I found the talk very informative.