Alright.
1. Rent a affordable place for the day.
2. Hire cleaners for the day after.
3. Everyone should be allowed to come. No invites needed.
4. Everyone brings their own liqueur.
5. Buy 1 keg at least.
6. Music should be what everyone is listening to, not just you. Everyone should have a good time.
7. End it earlier rather than late, that why too late. Then you skip a lot of puking and girls too drunk crying about stuff (some guys as well).
8. Having a designated driver at hand always is a plus.
* Edit
I always have around 40-50 people here every summer.
Pic related one side of my terrace