In the ideal coup d'état, you have
tanks surrounding the
presidential palace, you close the main
airports, you seize the
television station, and have
jets flying low overhead. Some middle-ranking member of the new
military junta, perhaps a
lieutenant-colonel, goes on television in full
uniform and announces a
curfew, the suspension of the
parliament and the
constitution, and the formation of a National
Salvation Committee (or it might be a National
Revolutionary Command Council, or a
Directorate of
Public Safety). Then
patriotic songs are played continuously over the radio. There is no need to actually hurt anyone as long as you implement these measures correctly.
The term first came into English after President Louis Napoleon of France overthrew his republic and elevated himself to Emperor Napoleon III. -- See Noung's node below for the original use in French: it might have become more familiar in the modern sense for Napoleon Bonaparte's overthrow of the Directoire.
Three of the best coups, from a purely aesthetic point of view, happened in 1973, which I think was a real highlight for the coup d'état. The overthrow of Allende; that of Papadopoulos by his fellow generals; and that of King Mohammed Zahir Shah by his cousin in Afghanistan. This last one had the added bonus that the new ruler could abolish the monarchy and proclaim a republic. But of course this is a twist that can not be done every time.
Military coups (and frankly, my dear, who gives a farthing for any other kind) are not new. In the colony of New South Wales, precisely on the twentieth anniversary of its founding, there was a full and correct military coup d'état. The armed forces overthrew the governor, Captain Bligh, and surrounded the governor's palace with artillery, and Lachlan Macquarie was proclaimed "lieutenant governor". (This event is also known as the Rum Rebellion.)
In England in 1658 to 1660, after the death of Cromwell, there was a series of military coups, with each general moving in troops to take control of London: first General Fairfax, then General Lambert in late 1659, then General Monck, who summoned the parliament that restored the King.
And as for the Roman Empire...