File this tip under "obvious yet important": when you're throwing a dinner party, don't forget to put out water.
It's easy to get caught up in the cocktails, or wine, or beer, or whatever special drinks you're serving. At our house, people usually get started on their first drink before they even sit down. But making sure there is water on hand--even if people don't want to drink it right away--is really important. Otherwise, no one will get hydrated, they'll just get crocked. And you don't want a bunch of overserved friends on your hands. Trust me.
Instead of having to keep an eye on everyone's water glass like a waitress, I just put a pitcher or carafe of water and some glasses out where everyone can see them. I like using a glass lemonade bottle, which I washed to remove the paper label. It can be resealed and adds a certain charm to the table, even though it's just an old bottle, otherwise destined for the recycling bin. (This bottle also works really well for transporting drinks to other people's parties or for picnics.)
Totally a no-brainer but something I often forget myself!