Hold each of these 20 people responsible for the sale of their allotment of tickets. Or, the dealer could be dressed in the sponsor uniform. Make your sponsors feel as though they are getting value for their donation and not only are they more likely to attend the event, getting a similar sponsorship the next year will be much easier. Ticket prices and what people are getting for their money will generally determine whether guest's drinks are included in the ticket price or if they need to pay for them. Typically, the more expensive the entrance fees the less likely you are to charge additional for drinks.
Additional drinks require the purchase of more drink tickets.
Manage your bar wisely. This follows a similar format to your drinks. However, don't spend all your money on providing a spectacular meal because that is not the focus of this type of evening. However, they do require a lot of time and effort to coordinate successfully. Delegate at least one person whose sole responsibility is to manage the silent auction of the event.