actually, the hidden messages are in the shows/movies themselves, rather than in the commercials.
One method I know of is inserting a single frame of the product. Coca Cola used it a while ago in theathers.
During the playback of the movie they pasted a single frame of a bottle of coke - you are exposed to it for only 1/24 of a second. this enters your sub-conscience, and sales of coke bottles skyrocketed.
this method is illegal nowadays.
today people just advertise the product. in the movie "Ace Ventora - pet detective 2", Ace drives through Africa and sees a big "Subway" sign.