Make sure you have a signed letter from your vet stating how many cows, calves and bulls you have. If you don't have it, they probably won't let you accross with it. They will let you bring enough back to do the total # of head that you have (at cow - bull sized dosages)
You cannot bring Ivermectin back for your friends, so either don't or bring it back under your own name.
Other than that, it is pretty straight forward, just go down and buy it wherever you want. They will allow some other products accross, but check before you go if you are planning that. Vaccines are a no go. Oxytetracycline and such you can (or at least they have let us) bring.