The “How to Network at a Conference” post on WikiHow provides a great approach as to “How to Excuse Yourself Gracefully. http://www.wikihow.com/Network-at-a-Conference
The Positive Comment
“Steve, it was really a pleasure speaking with you about [Topic]. I’m going to take a look at some of the other exhibits here, but if I don’t run into you later, I hope to see you at another event soon.”
“Please let me know how that project goes, I’d love to see it and hear how it turns out.”
“Have you seen anyone from [company name] tonight? I’ve been meaning to chat with them.”
“I need to use the bathroom, but I'm sure I'll see you around.”
(Just don’t walk ten feet away and start talking to someone else, your former conversation partner is going to know you lied and dumped them)
“I don't want to take up all your time; it was great meeting you!”
Bow out when others join the conversation.
This is a standard, tried and true method. Once other people join the circle of conversation and things get going between your old partner(s), you slip away.