One of the hardest things to make with clay are flat pieces. 

They need to dry flat and clay does not want to dry flat. Clay (no matter what type of clay you use), has a memory, so if you have been manipulating the clay, maybe raising the borders, the clay will try to go back to that state. 

Even if you do not touch the clay at all, a flat piece will tend to warp as it dries. If you are making a wall hanging or a plate or a cutting board, this may make your pieces wobbly and unusable.

There are few ways to prevent warping. The most important thing to do is to ensure that the piece dries flat. This is what I do when I make a plate (the same can be applied to anything that needs to be flat):

1. Depending on what you make, err on the side of making your piece a little thicker. 
2. If you are using a form, I never let it dry on the form. 
3. When the piece is ready to dry, I place fabric bags filled with rice on the piece, ensuring that it covers the whole surface. The key in drying, especially if you are going to be firing the piece, is even slow drying. So I always place the plate under a plastic sheet, away from a draft or current.
4. I do not remove the bag until the piece is leather hard and ready to finish / trim.
5. I test if the plate is wobbly. 

Hope this helps! This will be applicable to any type of clay, even air-dry clay.