Do everything you would normally do for a party, just backwards.
On the invitation, ask guests to wear their clothes backwards.
Think about how you would normally decorate, then do the opposite. Hanging balloons? Forgo the helium and hang them upside down from the ceiling. Hang banners from right to left, and have guests write their names backwards on name tags.
Arrange place settings underneath the table, and move the chairs aside for a game guests can play after the meal.
Ask people to enter through the back door, and greet them when they arrive by saying “Good-bye.”