Lessons learned:
Buy it early.
Don't just wear random blood in your face -- people will ask you if you are hurt.
Ensure that you are comfortable and warm, as (at least in Berlin), you may stand in the queue before a club for some time.
If wearing a robe, ensure that it does not drop down to the floor (really annoying when peeing/washing your hands/walking through the rain and puddles).
If wearing a mask, ensure you can breathe (and if wearing glasses, that the glasses don't fog).
Wear your costume with pride -- put it on before leaving the house. I usually need some time to get into the 'party' mood, and wearing the costume early helps a lot. In fact, this Halloween I started very shy, not daring to put on my robe+mask at all. But as soon as I had everything on I loved it so much, I even wore the costume the day after, scaring a few people 😁.