You could keep updating the weight you give to each upvote. That would mean upvotes you make at the start of each month are valued too highly before getting valued less, but as long as the creation date of all the accounts are distributed evenly across the month that should even out.

Alternatively you could base it on the number of upvotes a user gave last month, before correcting it at the end of the month, although that system might be easy to game.