From the Wikipedia entry:
Memes, analogously to genes, vary in their aptitude to replicate; successful memes remain and spread, whereas unfit ones stall and are forgotten. Thus memes that prove more effective at replicating and surviving are selected in the meme pool.
Memes first need retention. The longer a meme stays in its hosts, the higher its chances of propagation are. When a host uses a meme, the meme’s life is extended. The reuse of the neural space hosting a certain meme’s copy to host different memes is the greatest threat to that meme’s copy.
A meme which increases the longevity of its hosts will generally survive longer. On the contrary, a meme which shortens the longevity of its hosts will tend to disappear faster. However, as hosts are mortal, retention is not sufficient to perpetuate a meme in the long term; memes also need transmission.
Life-forms can transmit information both vertically (from parent to child, via replication of genes) and horizontally (through viruses and other means). Memes can replicate vertically or horizontally within a single biological generation. They may also lie dormant for long periods of time.
Memes reproduce by copying from a nervous system to another one, either by communication or imitation. Imitation often involves the copying of an observed behavior of another individual. Communication may be direct or indirect, where memes transmit from one individual to another through a copy recorded in an inanimate source, such as a book or a musical score. Adam McNamara has suggested that memes can be thereby classified as either internal or external memes (i-memes or e-memes).
Some commentators have likened the transmission of memes to the spread of contagions. Social contagions such as fads, hysteria, copycat crime, and copycat suicide exemplify memes seen as the contagious imitation of ideas. Observers distinguish the contagious imitation of memes from instinctively contagious phenomena such as yawning and laughing, which they consider innate (rather than socially learned) behaviors.
Thought Contagion
As noted in Aaron Lynch’s 1996 book Thought Contagion: How Belief Spreads Through Society (New York: BasicBooks, p. 208, ISBN 0-465-08467-2), the author identifies seven general patterns of meme transmission:
1. Quantity of parenthood: an idea that influences the number of children one has. Children respond particularly receptively to the ideas of their parents, and thus ideas that directly or indirectly encourage a higher birthrate will replicate themselves at a higher rate than those that discourage higher birthrates.
2. Efficiency of parenthood: an idea that increases the proportion of children who will adopt ideas of their parents. Cultural separatism exemplifies one practice in which one can expect a higher rate of meme-replication—because the meme for separation creates a barrier from exposure to competing ideas.
3. Proselytic: ideas generally passed to others beyond one’s own children. Ideas that encourage the proselytism of a meme, as seen in many religious or political movements, can replicate memes horizontally through a given generation, spreading more rapidly than parent-to-child meme-transmissions do.
4. Preservational: ideas that influence those that hold them to continue to hold them for a long time. Ideas that encourage longevity in their hosts, or leave their hosts particularly resistant to abandoning or replacing these ideas, enhance the preservability of memes and afford protection from the competition or proselytism of other memes.
5. Adversative: ideas that influence those that hold them to attack or sabotage competing ideas and/or those that hold them. Adversative replication can give an advantage in meme transmission when the meme itself encourages aggression against other memes.
6. Cognitive: ideas perceived as cogent by most in the population who encounter them. Cognitively transmitted memes depend heavily on a cluster of other ideas and cognitive traits already widely held in the population, and thus usually spread more passively than other forms of meme transmission. Memes spread in cognitive transmission do not count as self-replicating.
7. Motivational: ideas that people adopt because they perceive some self-interest in adopting them. Strictly speaking, motivationally transmitted memes do not self-propagate, but this mode of transmission often occurs in association with memes self-replicated in the efficiency parental, proselytic and preservational modes.
Full version with cited sources: https://en.wikipedia.org/wiki/Meme