Purchase special padding to place between the area rug and the carpet. Though the padding may seem as though it will make the area as a whole too padded, special rug pads are made specifically for this purpose. These may be found at flooring stores. They help the rug lie correctly, but also prevent any color runoff that could occur from the rug onto the carpet.
Use self-adhesive tape to keep the throw rug in place. Use double-sided tape and apply it to each of the edges of the throw rug prior to placing it onto the carpet. This typically works better when attaching the throw rug to hard surfaces, though it can be used on carpet, especially carpet that is not extremely plush.
Replace the rubber backing entirely that is on the back of your current throw rug. If the rubber backing has worn away, it can cause the rug to slip more on top of the carpet. New rubber backing slips can be purchased at flooring stores.
Place additional rubber on the back of the throw rug. If you have any rubber rings, for example, that come from canning jars, attach these rubber rings to the bottom of the throw rug. This extra layer of rubber may help keep the throw rug from sliding. To attach the rubber rings, sew or glue them on.