So, given that my social energy is limited, here's what I do:
* Have a concrete goal for each event, things like: small talk with 5 random people, elevator pitch to Jane, get contact details to follow up with Jim and Mo, etc.
* Step out of unproductive/negative chats asap - "excuse me, I need to make a call/ check email / catch Bob before he leaves"
* For longer events, take regular smoke breaks (even though I don't smoke)
* Leave (politely) as soon as goals are achieved or as soon as I'm worn out