This little tip is easy, if you're the one opening a new package of corn starch (or any other dry type of material, which is in a plastic or cardboard container and has a security seal on it).
Instead of just ripping off the seal entirely, cut it in half so that it's still covering half of your container; this gives you something to level off the measuring spoon, ideal for corn starch, ice tea mix, cocoa powder and the like.
I use a serrated steak knife to cut the seal in half, so it gives a good edge.
Scissors won't cut it properly, so use an Exacto or craft knife, sharp razor knife or something similar.
This will give you a way to use an accurate measurement with a measuring spoon or teaspoon, by using the edge of the seal to scrape off the excess powder from the spoon, giving you the perfect level measurement.
Corn Starch isn't the only thing to come packaged like this. Any kind of powdered dry goods that the manufacturer offers in the same kind of container, safety sealed with a paper or plastic cover, is a good candidate for the same treatment.
There are even wet products that it works on too; I use this method on Kalamata olives and feta cheese, which makes for a lot less mess.