There are a few things to keep in mind when making a flat tile. Keep it laying flat while it is drying. Try not to bend the soft clay tile when moving it from your work space to the area where it will be drying. Bends in the soft clay often redevelop later in the process. If you're cutting your clay from a pugged block, be sure to cut from the sides of the block, not the ends.
Thicker tiles tend to warp less than thin ones.
This is, at least partially, because they tend to dry more slowly. Clays with grog, fireclay or other non-plastic materials tend to warp less than smooth, fine particled clays. Do not try to force dry tiles that have shown a tendency to warp using a fan or heater. It is better to try to slow down the drying process with a thin sheet of plastic. Good results have also been obtained by stacking tiles up to four or five high with alternating layers of sheetrock.