PET bottles are generally used for soft drinks and Gatorade. They can be frozen and sit out in the heat all day without damaging the contents inside or warping the bottle itself. With a little bit of cleaning, these bottles can be reused multiple times before getting thrown away.
Remove all the labels from your bottles before cleaning them. You can do this with scissors or by soaking the bottle in warm soapy water to dilute the glue. The method you use will depend on the actual labels. For example, soft drink bottles have labels that are only glued in one spot. Using the scissors, you can easily cut the band and then unwrap what's left.
Unscrew the tops and set them in a container of warm soapy water to avoid losing them down the drain.
Fill a large pot or sink with soap and hot water. You'll want to fully submerge your bottles into the solution for a few minutes to kill any bacteria. If scratches are on the inside of the bottles, recycle them. Bacteria can become lodged in those scratches and multiply later.
Rinse the bottles and tops thoroughly. For the bottles, fill them with warm water from the tap until no soap residue is left over. You can also place the caps on the bottles and shake the water inside to see if soap bubbles appear. If they do, rinse again.
Stand the bottles upside down on a well-ventilated drying rack. Don't lay them on their sides as they'll take longer to dry and may dry unevenly. Allow the bottles to dry overnight. Don't refill the bottles too soon; they must be completely dry before refilling to avoid bacterial buildup.