When your guests overstay at a party that is certainly a kudos to the host. That means your planning allowed for perfect mingling. When building a traffic pattern use a three-to-one guests-to-seats ratio. Count all the places one could sit; sofas, rented chairs, ottomans, even wide windowsills. Aim for three people for every available perch so you have a good number of folks moving around. Also, consider renting Bistro tables that urge the crowd to stand and converse. Another way to ensure swirl is to keep the Food table and Drinks on opposite sides of the party area. Distribute three or four bowls of munchies (nuts, olives, pretzels etc.) around the room to ease the crush at the hors d’ oeuvres table and encourage guests to spread out. Provide a conversation Kickstarter. You can print conversation cards (find them online) and have them in a jar at the entrance. Ask everyone to pick a few and use them during the evening.