Use accessories like napkin rings, a centerpiece, candle holders or a table runner to indicate the theme of the party. If the themes colors (green for Saint Patricks' Day or red for Christmas) do not go with your rooms color scheme then use neutral accessories that look good with your decor. You can still make it special and festive by using fun, over the top glittering accents you would not use everyday. Or use natural items that are appropriate for the occasion like dried leaves and acorns for Thanksgiving or rose petals for Valentines day,
One way to change the look of a plain tablecloth is to add a transparent, shimmering organza overlay;or sprinkle sequins or glitter on top of the cloth. The idea is to make it special and different from what you do every day.