Every party has that guest who likes to hover over the host, asking if she can help. Extra hands in the kitchen are always welcome—unless the extra hands come with endless questions about where to find utensils and how you want things done.
Anticipate the helper with a special-projects station. Clear off a remote section of the counter ahead of time, stashing small appliances you aren't using (toaster, blender, juicer). Set out all the tools needed to do an easy-to-accomplish task, like cutting vegetables, arranging a cheese platter, or washing prep bowls. Stock the station with a cutting board, knives, a towel, an oven mitt—anything needed for the job. When the helper offers her services, say "Great! I could really use a hand over here." Then give the helper an apron, tell her she has total creative control, and walk away. You'll be left alone to do your work, and the helper will feel like she's been able to give you some relief.
If it's kids who want in on the action, give them a task that will keep them out of the way of the knives and the range. Ask them to fold napkins and set the table. Hook them up with crayons, notecards, and stickers to make place cards and menus for the table. If they insist on cooking, bring them in to stir as you add flavorings. Then give them all the credit the when the guests start oohing and aahing.
Last-minute shopping items: Crayons, cards, and stickers for the little ones