Let's say I make a spaghetti-and-meatball supper, and I made the spaghetti an hour or so before the meal. What's the best way to keep it fresh and warm until we're ready to eat?
You don't want to keep it warm -- that will lead to it steaming itself and overcooking. You need to get it cold and reasonably dry as quickly as possible so that it will stop absorbing water for the hour that it is sitting around, then reheat it quickly at the last minute.
Undercook the pasta slightly -- by somewhere between two and one minutes -- so that it is not crunchy anymore, but is extremely chewy. Pull it out, saving the hot water, and plunge it into as large a quantity of ice water as you can collect. As soon as the pasta is cold, pull it out of that water, too.
The surface of the pasta will still be covered in wet starch, which will cause all the pieces to start sticking together as they sit. To avoid this, toss the pasta with a little oil (it doesn't matter what kind you use, because you're going to wash it off later), so that all the pieces are coated. You can keep the pasta out on the counter if you're going to use it soon,* and you may want to cover it. When you are ready to eat, bring that cooking water back to a boil, and drop the pasta in. It will take a bit more than the subtracted minute to finish cooking; it depends on how quickly you cooled it down, how thick it is, how much you dried it off, how cold it is when you drop it back into the water, ... as always, tasting it is the only way to tell if it's done.
If you do put it in the fridge, it should be reusable for at least a couple of days.
I'm not sure why you want to cook your pasta ahead of time, but if your idea is to shorten the last-minute step of getting the pasta fully cooked, you should look into pre-soaking the pasta, a totally
heretical fascinating proposal that Harold McGee recently made.
*Food-safety-minded people would say you need to refrigerate it if you're going to keep it for more than two hours.
As Kyri says, make the pasta just in time (JIT). But if you can't, you should refresh the pasta either by placing it under running cold water (not ideal) or in a cold Bain Marie with olive oil or butter. Once cold and still loose, you can keep it in the fridge for a day or two.
To reheat, add olive oil or butter to a skillet and when that's hot, add the pasta. Stir until it's warm. Add warm 'salsa' to the pasta in the skillet and mix.
I'd have to agree that olive oil on the pasta after it's cooked keeps it unclumped. A lot of people might say to cook with the olive oil, but if you keep it in the colander and toss the oil it will keep for awhile. The best thing to do is make the sauce and meatballs first, cooking the pasta at the very end. With most sauces, if you let them simmer awhile, you'll get a nice flavor, so you can keep the sauce on low while finishing with the pasta