Most packaging instructions for laying tiles will tell you to start with a perfectly flat and even surface, to ensure that the tiles are flat and even in the end. Like many home project instructions, this is often easier in concept than reality. Plaster walls, concrete or wood floors and other surfaces over which you might want to lay tile are very often humped, indented and otherwise uneven, which will prevent the tiles from setting evenly with each other. There are several ways to deal with this dilemma.
Installing cement board
The best solution to an uneven tiling surface is to cover it with an even surface. Cement board (a much harder version of plasterboard) is the standard today for laying most kinds of tile, and should be used for tiling whenever possible. Install 1.2 cm (1/2 inch) cement board for floors and 6 mm (1/4 inch) for walls. It's installed with thinset mortar and screws, and arranged in staggered rows, like plasterboard. Fill in the seams between the boards with thinset mortar and scrape them smooth to get a solid, flat, unbroken surface for your tile.
Liquid levelers and extra mortar
Sometimes you can't install cement board, because the surface isn't conducive to it or it will raise the level of a floor too much -- one example would be an existing tile floor that you want to tile over. In those cases, a combination of a liquid leveller and a thick mortar bed can solve the problem. Levellers are mixed to a watery consistency, then poured over the surface to settle into the dips and cracks, eventually drying rock-hard. Using a thicker layer of mortar than usual to lay the tiles will also help obscure the unevenness of the surface.
Surrender to unevenness
Sometimes no amount of preparation work will fix an even surface in preparation for tiling -- a patched-up, wavy concrete patio, for example. At that point, it may be time to invoke an old carpenter's axiom: if you can't fix it, make it look like you planned it that way. Many types of tiles are made to look random and uneven, including tumbled travertine, Mexican saltillo and roughcut flagstone. When installing these tiles on an uneven surface, it's still a good idea to use extra mortar, so you don't get air pockets underneath that could loosen them.