Create some sort of request intake form with all the details you need.
Highlight very clearly what your lead-time requirement is.
Determine what types of events you can support. For example, my team supports tier-1 and corporate events, but we do not support smaller regional events -- our regional marketing team owns those but keeps us in the loop on any customer speakers they source in partnership with the local sales team. We simply do not have the bandwidth to support all the regional events our company participates in.
For tier-1 and corporate events that we do support, we require a 2-3 month lead to for customer sourcing / content creation and prep time.