Brush your cat often. A well-groomed tangle-free coat will help to keep your cat cool.
If your cat has long fur, consider professional clipping to help ease their suffering during summer. White and light-coloured cats also have very pale skin and can be subject to sunburn. Talk to your veterinarian about whether giving your cat a clip could help to keep them cool in the hot weather. Another option is to have only the tummy clipped, which offers sun protection as well as cooling.
You may also want to try taking a damp washcloth or paper towel and stroking your cat with it. Most cats don’t mind a little bit of moisture on their fur, especially when they notice how it can cool them off. In fact, one of the ways cats cool themselves down is by grooming, which is nothing more than wetting their fur with saliva rather than water.