Reduce the list before voting by combining similar items. This prevents a big vote being hidden, as several people vote for the same thing in different places.
Multivoting involves reducing the list in several steps, where:
Each person gives one vote each to one third of the items on the list.
Only items for which at least half the group voted are carried forward.
This is repeated until a manageable number of items is left.
Have separate sets of people generating the list and voting on it. This ensures voting will be a more objective exercise.