what are the best precautions for trappers to protect against rabies
After you know the animal is dead, remember that the virus lives on in the brain and saliva. Wearing gloves when skinning, good gloves not the cheap thin ones that tear easy. If you are going to mess with skulls, then let them sit in warm temps for several days.
I do collection for testing and the preferred way to preserve the virus for testing in the brain is to freeze, so don't think that freezing will do anything. Actually warm temps will kill the virus in the brain once the animal is dead.
Avoid scratches and if you have any open sores make sure they are well covered.