First of all, get used to check if your clothes are smelly or stained before dumping them into the laundry basket. Wash less often, make sure your clothes smell clean and you will be saving energy, money and time. This way you also can be sure that the load is full and you will be taking full advantage of your washing machine.
Before washing your clothes, remember to pre-treat the stains on your clothes as soon as you need to dump it in the laundry basket. This will make easier for the stains to disappear and it will let you use colder water in the washing process. There exist a lot of different tricks to get rid of stains with natural products!
Choose natural clothing fibers over synthetic ones. Polyester, acrylic or nylon when washed leave behind small particles of microplastic. Recent research proved that the oceans and the animals that inhabit it contain microplastics, they are present in the fishes we eat and the problem is already severe. Choose organic cotton or wool, natural fibres in general to stop polluting the oceans (and if its fair trade, you won’t be contributing to inequality).
Don’t buy clothes that require dry cleaning, the chemical used in the process is linked to cancer and other health problems. There exist alternatives that use different chemicals that doesn’t harm the planet, but it will be easier just to avoid it.