A great tip for keeping flies at bay, cut a lemon in half and add cloves to it. Set out in bowls about 30 min before food will be going on the table. The smell with leave your party pest free (at least the little kinds).
To keep flies away - Fill a zip lock bag with water and 5 or 6 pennies and hang it in the problem area - apparently, it keeps flies and wasps away. Some say that wasps and flies mistake the bag for some other insect nest and are threatened.