Q. I have read there is a substance you can add to whipping cream to maintain its firmness. My cream doesn't keep firm for long even though I use powdered sugar for sweetening and follow all the general rules.
The substance you're thinking of may be corn syrup. Adding 1 teaspoon of light corn syrup per 1 cup of cream before whipping can make the cream more stable. Unflavored gelatin is sometimes used, too, although I find either corn syrup or gelatin can give the cream a gummy texture.
You're right that using confectioners' sugar can help stabilize whipped cream. That's because confectioner's sugar contains a small amount of cornstarch. It also will help if you beat the cream until it forms soft peaks (when you lift the beaters, the peaks stand up but curl over) before you add the sugar. Also, make sure you're beating the cream to stiff peaks. For a fast check, tilt the bowl. The cream should stay in place without moving.
To whip cream in advance, I use a couple of tricks. To whip it several hours in advance, spoon the whipped cream into a bowl-shaped sieve lined with a coffee filter or two to block the holes. Place the sieve in a bowl and refrigerate the whole thing. The cream will exude a little liquid, but most of it will stay fluffy. If it gets too stiff to use, whip in a little cream to loosen it up again.
You also can whip cream in advance, then dollop or pipe it onto a baking sheet and freeze it. Put the frozen mounds in a resealable freezer bag, seal it tightly and keep in the freezer for up to two weeks. You can place the frozen mounds directly on a dessert and let it thaw for 5 or 10 minutes before serving.