Subliminal messages can cause effects on a humans’ emotion or behavior but most effectively when done so unconsciously. The most extensive study of therapeutic effects from audiotapes was conducted to see if the self-esteem audiotapes would raise self-esteem. 237 volunteers were provided with tapes of three manufacturers and completed posttests after one month of use (Greenwald, Spangenberg, Pratkanis, & Eskenazi, 1991).
Subconscious stimulus by single words is well known to be modestly effective in changing human behavior or emotions. This is evident by a pictorial advertisement that portrays four different types of rum. The phrase “U Buy” was embedded somewhere, backwards in the picture. A study (Keys, 1973) was done to test the effectiveness of the alcohol ad. Before the study, participants were able to try to identify any hidden message in the ad, none found any. In the end, the study showed 80% of the subjects unconsciously perceived the backward message, meaning they showed a preference for that particular rum. Although every subliminal message may not affect everyone, it is obvious it does affect the behavior or emotion of most.
You can find more information on Subliminal Messages at http://www.subliminalmp3s.com, a site that offers free audio subliminal messages to assist changes to a person behavior.
Backmasking, an audio technique in which sounds are recorded backwards onto a track that is meant to be played forwards, produces messages that sound like gibberish to the conscious mind. Gary Greenwald, a fundamentalist Christian preacher, claims that these messages can be heard subliminally, and can induce listeners towards, in the case of rock music, sex and drug use. However, this is not generally accepted as fact.
Subliminal Messages in History;
In 1978, Wichita, Kansas TV station KAKE-TV received special permission from the police to place a subliminal message in a report on the BTK Killer (Bind, Torture, Kill) in an effort to get him to turn himself in. The subliminal message included the text “Now call the chief,” as well as a pair of glasses . The glasses were thought to be of significance to the killer because when BTK murdered Nancy Fox, there was a pair of glasses lying upside down on her dresser. So, police felt that the glasses would stir up some remorse emotion and included them in the subliminal message. The attempt was unsuccessful, and police reported no increased volume of calls afterward. 
Before the re-election of French president François Mitterrand in 1988, a subliminal picture of him was mixed in the title sequence of French national television daily news show, and it appeared for several consecutive days.
The subject was also prominently featured in the 1999 film Fight Club. Pictures of the main character, Tyler Durden, flash onscreen at various points during the earlier parts of the film, before Durden is introduced. Also, Durden is shown at his job as a projectionist, splicing pornographic flash frames into a film he is showing. A picture of a penis flashes before the end credits.
During the 2000 U.S. presidential campaign, a television ad campaigning for Republican candidate George W. Bush showed words (and parts thereof) scaling from the foreground to the background on a television screen. When the word BUREAUCRATS flashed on the screen, one frame showed only the last part, RATS. The FCC looked into the matter, but no penalties were ever assessed in the case
A McDonald’s logo appeared for one frame during the Food Network’s Iron Chef America series on 2007-01-27, leading to claims that this was an instance of subliminal advertising. The Food Network replied that it was simply a glitch.
In Formula One racing, the paint scheme of many cars would carry messages intended to look as if they were of banned tobacco products in many Grands Prix where tobacco advertising was banned, though many of these were jokes on the part of the teams (for example, Jordan Grand Prix ran Benson and Hedges sponsorship as “Bitten and Hisses” with a snake-skin design on their cars). A similar procedure was used by NASCAR driver Jeff Burton after the AT&T Mobility advertising was banned by a court order in 2007.
On November 7, 2007 Network 10 Australia’s broadcast of the ARIA Awards was caught out for using subliminal advertising in an exposé by the ABC (Australian Broadcasting Corporation). 
In June-July 2007, Sprite used a type of obvious subliminal message, involving yellow (lemon) and green (lime) objects such as cars. The objects would then be shown inconspicuously in the same setting, while showing the word “lymon” (combining the words lime and lemon) on screen for a second at a time. They called this “Sublymonal Advertising.” The previous year, Sprite used a similar advertising campaign, but this time it was tied in to The Lost Experience, an alternate reality game.