When using such devices, you must practice the double, because it involves concealing something extra. For example, you may use a piece of transparent tape for doing a card trick. The piece of tape is your gimmick. You can imagine how difficult will it be to do a card trick with a piece of tape. That's why you must practice so your audience can not see the tape.
Misdirection is also very important in magic.
Misdirection is to direct audience eyes and minds to one place or trail. For example, when vanishing an object, you take the object in one hand and place it in the other. There the process of misdirection starts. You have to make people believe that the object is in your hand and that you're doing magic movements, and throwing magic powders and blowing. If you're not looking at your pocket, they will not be inclined to look at it, unless you look at it.