For starters, don't buy the biggest bird that will fit in your oven. Buy something that is sure to feed your guest list and leave a little behind. Let's face it, we do want some leftovers, just not enough to feed an army!
Secondly, easy on the sides. We all love them, but the traditional sides are often not figure flattering or heart healthy. Make fewer sides and in smaller portions. Consider sliding in a green salad for bulk!
Third, If you do make the prize bird and all the fixings, plan to have freezer worthy storage containers ready. Invite your guests to bring some containers with them. Then there will not be any tupperware tracking when you need your favorite container for a party in July! (Here is a great cheat sheet from the USDA about how to handle leftovers ie freezing, reheating, refreezing, etc. http://www.fsis.usda.gov/PDF/Leftovers_and_Food_Safety.pdf )
Finally, invite guests to bring their favorite leftover recipes on index cards to share with other guests. Imagine heading home after Thanksgiving dinner with a package of delicious goodies and some new recipes to try out. Variety is the spice of life!