This is an interesting question because so much of the imperialist effort of the West occurred before the nineteenth century and industrialization. By the middle of the eighteenth century, Europeans had already conquered (and then lost) the Americas. The British had taken over India, the West Indies consisted of British and French colonies, the Dutch had established outposts in the East Indies, and the British had landed in Australia, all before 1800.

In the following century, the main expansion involved the opening of China and Japan to trade and then taking over the African continent. It would seem simplistic to say these goals were accomplished because the Europeans had better weaponry and ships, but it is mostly true. The ability through industrialization to mass-produce weaponry was, of course, a major factor. So were the invention of steamships and the railway, which made it possible to transport supplies more quickly and efficiently for military use. The invention of the telegraph allowed instantaneous communication in coordinating military, trade and construction efforts.

Both China, which became a quasi-client state of Britain, and Japan were intimidated by superior weaponry and the evidence of the results of industrialization in the West to open their ports to European and US trade. Finally, the building of the Suez Canal, which would have been impossible in a non-industrialized world, facilitated the rapid transport of supplies between Europe and Asia and thus a stronger grip on the existing colonized areas, plus the conquest of further regions in Asia such as Burma (by the British) and Indochina (by the French).

In general, these countries used two types of tactics.

First, they used economic tactics.  The industrialized countries could use their economic power to come in and take effective control of the smaller countries that they wanted to control.  A good example of this is how American sugar planters took economic control of Hawaii before Hawaii actually became part of the United States.

Second, they used their military power.  A good example of this is how England used military power to defeat China in the Opium Wars.  This allowed England to get control of Hong Kong and to gain other concessions from the Chinese.