New fabric of all kinds contains residual chemicals from the manufacturing process. These can cause stains to 'set' more than they would on laundered fabrics. For this reason it is best (if possible) to launder your chair covers before their first use to minimise the chances of this happening.
Pre-treat stains. Polyester is tough and offers good stain resistance but oily stains can become 'set' by heating in the laundry process and all stains are best dealt with by pre-treating. Experiment with different pre-treatments (test on a small inconspicous area first). Many of our hire company customers report good results using Vanish sticks. White spirit can be effective at removing shoe polish.
For the laundry use a synthetics cycle keping the temeperature and spin speed slow. Don't overload the washer. The covers should be able to move around freely. You will find you can get away using a lot less washing powder than for other load types. High temperatures and high spin speeds (especially if combined) can cause the filaments in the polyester to be damaged which can result in permanent creasing. With correct repeated washing the anti wrinkle & crease resistant properties of your polyester chair covers should actually improve.
It is essential to ensure you do not use too high a temperature to avoid the risk of permanent loss of the wrinkle and crease resistance properties of your polyester chair covers. In order to get chair covers that are ready for use without further pressing it is essential that the dryer is not overloaded and that the covers can drop freely from the top of the tumbler drum. Despite using a low spin speed in the wash, the non-absorbent characteristics of polyester means the fabric should retain little water and will dry quite quickly. Ideally you want the drying cycle to complete while there is still just a tiny amount of residual humidity. Auto sensing tumblers can help in this. It is vital that the chair covers are removed from the dryer as soon as the drying cycle has completed. A pile of warm chair covers sitting in the bottom of the dryer will quickly get creased and wrinkled.
Ideally you should hang up the chair covers using soft grip hangars as soon as they are removed from the dryer. This helps to avoid wrinkles and creasing. If you need to store them in a box make sure they are completely dry first (to avoid mildew damage and minimise creasing) and do not pack them over tightly (which again will cause creasing). If packed fairly loosely polyester chair covers can generally be kept in cardboard boxes for 2-3 weeks before some creasing may start to show.