A good way to build sales within the four walls of the restaurant is to run server contests that focus on selling items that can increase your check average, help get rid of high inventory items before they go bad (this works great in the case of over ordering which also helps combat high food cost), help increase sales in an area that may need a little help such as wine or liquor, and can also help build sales on slower nights.
If you provide great contest prizes this can have a huge impact on employee morale and can help generate competition amongst the servers and once everyone is on board it can be fun and everyone wins.
Here is a list of some on the contest I have done or seen done in the past:
Bingo-fill out a sheet of paper with a bunch of squares listing the menu items or drinks you would like sold. The first one to complete it wins.
High category sales, such as appetizers, wine, dessert etc…
Highest check average.
Wine feature contest for a month based on highest sales. Usually, three winners; first, second and third place.

Here are some the prizes I have offered in the past.
No closing duties or side work.
They get to write their own schedule for the next week.
Trade outs with other restaurants or local businesses.
Beer, wine and liquor purveyors can also give some really good prizes.

Anytime you can get your employees to compete with each other, can create a positive fun atmosphere where they can win some really cool prizes it a great morale builder.