Bring platform shoes in the size of your actor to the set. If a male will be wearing them, you might also need a pair of extra long pants to cover the added height of the shoe. For women, a long skirt or dress may be used to hide the platform.
Insert two-inch padded soles into the actors' shoes if you cannot easily hide platforms. Although this only adds a few inches in height, it is more discreet and less expensive than other techniques.
Ask the actor to stand on a step stool in scenes where his feet will not be seen. For example, a bartender standing behind the bar can easily become taller by standing on the stool. This can also work for scenes where the actor is behind a podium or counter.
Construct undersized doorways for shorter actors to enter through. By doing this, actors will appear taller on screen. Standard sized doorways measure 80 inches, so you can choose to lessen that according to the effect you want to create.
Use a ladder so that the cameraman can enter the room from a high position, which will make actors appear taller.