Keeping tiles flat while drying and firing has often been a source of frustration for clay artists. Over the years, I've read a great deal about sandwiching wet tiles between drywall, flipping them, stacking them, turning them, covering them or weighting them.
Why spend countless hours fussing over tiles? I've developed a technique that greatly minimizes the amount of handling needed and is almost foolproof for making flat tiles.
To begin making flat tiles you need to use a heavily grogged clay formulated for sculpture or tile - not a plastic throwing clay. I like my clay on the dry, stiff side as too much water makes it dry slowly and promotes warping.
Most of my tiles are press molded in plaster molds, but if you don't use molds for your tiles, just roll out clay slabs directly onto a piece of drywall (drywall makes a great work surface - just make sure to seal all of the drywall edges with duct tape to contain that nasty drywall dust) using wooden spacers or dowels beneath the rolling pin for the desired thickness. I prefer half-inch-thick tiles.
Once you have rolled out the clay slabs, don't move, lift or turn them. If you do move the clay, its "plastic memory" will kick in and it may warp, bend, or curl during drying and firing. Just trim the slabs in place, cutting them to the desired dimensions using a trimming knife and your pattern. After trimming, it is very important to allow the wet tiles to sit on the drywall for 8 to 12 hours (overnight is usually good). Drywall sucks a lot of water out of the clay and the tiles will really stiffen up.