Anyway, once they are taken out and installed/put on, allow a week or three and the lines will eventually relax and slowly straighten out.
If you'd like to accelerate this process, there are several options depending on the tools at hand:
Iron: Set between warm to medium setting and simply run on the surface, but iron it over a piece of fabric (like a sheet or a towel); instead of applying direct heat on the fabrics. While natural fabrics will be able to accept the direct heat, make sure you DO NOT IRON DIRECTLY on Synthetics and Leather Slipcovers.
Hand-held Steamer: You can also use a Hair-Dryer and Water Mist Sprayer instead of a hand held steamer; simply run them through the wrinkled area adequately and voila, you'll have a wrinkled free slipcover. If you're using a modern iron with a steam function, use them against the back of the slipcovers.
To store your slipcovers, try to use the cloth bag provided to allow the slipcovers to breathe. Plastic packaging/vacuum storage will likely to cause mildew or molding. (Natural Fabrics and Leather are more vulnerable).