Paraphrasing something I wrote some time ago:

The Clearances fall into three distinct stages. The first stage began with the introduction of sheep farming to the Highlands from 1760 onwards and ended with the establishment of the large sheep runs in the interior of the country and the people on the coast.

In this first phase, the Tacksmen were the primary drivers of emigration. The Tacksman as a social strata (they served both as military leaders and as administrators for the Clan Chief) had become obsolescent after the 1745 rebellion failed and the Clan system was dismantled. One factor would collect the rent and administer the land at less cost to the chief than the many Tacksmen could. Some Tacksmen became officers in the new clan regiments which were being raised at this time, while others took up administrative positions in the Empire. Others simply emigrated to America or Canada taking whole townships with them.

The second stage in the 1840s and early 1850s were intended to clear the land of those remaining people who were so destitute that the landlords could not support them. It was thought that they would have a far better chance of surviving outside Scotland than by staying at home.

The third stage of clearances was paid for by the landowners who found it cheaper to pay for the transport of their tenants across the Atlantic or to Australia. In many cases the tenants had no choice but to emigrate, their homes having been torn down to make way for sheep-walks. With nowhere left to go, the offer of passage to the colonies where they would be able to acquire land denied to them in Scotland was the only choice.

By the 1850s the Clearances were effectively at an end, for several reasons, firstly there were no more people to evict, secondly the population had finally begun to decrease, thirdly the economy was now beginning to pick up and finally the fishing industry was finally entering an upturn. Moreover the crofters were finally beginning to stir themselves on their own behalf with the final end being the passing of the Crofters Act in 1886

The early clearances were almost always from the land to the coast simply because at the time when wool prices were rising the prices for kelp (used to supply the raw materials for gunpowder during the Napoleonic wars) were rising too. Kelping was labour intensive and could soak up the excess population now created. Fishing was also put forward as a means by which the Highlanders could raise money. The removed tenant was given a small piece of land- the croft. If this land was bad- it was often the land which even the sheep farmer wouldn't touch- the crofter was forced into kelping. If the land was relatively good the crofter had to pay a very high rent and was therefore forced into kelping.

For the Clan chiefs it soon became apparent that in order to fund their new lifestyle in the south, they needed money. It cost more to have people on the land than it did to evict them or simply pay for them to emigrate.

Some Sources:
Bumstead J.R., The Peoples Clearance 1770-1815
Grigor I.F., Mightier than a Lord: the Highland Crofters' Struggle for the Land
Richards E., A History of the Highland Clearances, Vols. I & II
Cameron, A.D. Go Listen to the Crofters: The Napier Commission and crofting a century ago
Hunter, J. A Dance Called America
MacKenzie, A.S. History of the Highland Clearances