Dance! If people are up and having fun, they'll stay. We did nothing "special" at our wedding to get people to stay, but we paid the dj for an extra hour, and people were having so much fun that our venue basically had to kick people out. We didn't do "wedding" music, but stuff that people thought was really fun - we got a lot of comments about the AC/DC, but it was pretty awesome seeing our great-aunts and great-uncles up there rocking out : )
On a more practical level, keep rolling out food. So maybe don't have a cake, but an assortment of desserts that can keep being "replenished." Cupcakes would work. So if you put out 30 cupcakes on a buffet table, people will come up and eat, but then if you put out 30 more, some people will be convinced to stay for "just one cupcake" and other people might stay for a second : ) Probably a little more money, but food gets people to stay. It also avoids the traditional "we'll make our exit right after they cut the cake."
Good luck.