The best way is to keep them indoors. Mine like to lie on the deck during the day which is ok but I have to watch that they are not right in the sun and always have a water bowl with cool water close by. I will throw an ice-cube in the water throughout the day to keep it cool and get them interested.
Lily is a white cat with very pink skin. She has to have sun-screen put on her nose and ears or she can easily get skin cancer. I also don't let her lie directly in the sun.
Signs of heat-stroke in cats