Duct tape? :p

As already said, velcro and safety pins work... but you need to focus more on how you move your head around; try to minimize turning your head too much if your suit has okay visibility. Another thing to look at would be wearing a shirt with a relatively tight collar and or wearing an actual collar. Then again, depending on how the excess material is shaped, it may be inevitable. Perhaps look into altering the neck fur?

Alternatively, you could just leave it hanging out like most partial fursuiters do...