As I say all the time, mix things up. Mix grandma's china with flea market finds. This adds casualness, and keeps your guests from getting fidgety. Find a color palette that is appropriate for the time of year, and the purpose of the lunch. Spring is a great inspiration for luncheons. The trees are blooming, along with all those beautiful flowers, and they create an unimaginable backdrop of art. Coordinate your linens and dishes with this back drop and keep it simple. If you are unsure of what to choose, white and cream are fail proof.
Use simple containers from around the house for flowers. Quick tip: Use fruit as an easy arrangement. Zest a few lemons or oranges and place them with toothpicks in a small vase with floral foam to hold them in place. They provide great color, and smell wonderful. Place candle holders or lanterns on the table to create atmosphere. Make sure your candles are unscented, and remember to trim the wicks before you light them so they burn cleanly.
Make sure the table is pleasing. Keep things simple, but delightful for your guests. If you are entertaining more than six people, make sure staple items are accessible such as salt, bread, water, and butter. I generally have two of each, or one on each end of the table.
Create a dessert buffet table that displays your sugary sweets like a trophy. This table can be overly decorated and be prepared as eye candy for your guests. Offer guests glasses for water or wine, and additional plates so they can enjoy these extras at their own pace at the end of the meal.
Your food needs to taste good, but make sure it has the made-at-home charm. Use beautiful or unique serving dishes, and make sure the items don't look too perfect. If it looks complicated or too ornate your guests will hate to dive into it. (Remember forgo the fussiness)
Last but not least, choose a menu that is appropriate but easy for you to manage. The food has to compliment the table and vice versus. Create a menu that can be made ahead of time, so that when guests arrive, you're out of the kitchen and enjoying their company.