Dogs only sweat through their paws, so let them stand in shallow cool water, they often like to have their tummies and chest wet, so they may lay in it.
Supply plenty of clean, fresh water for them to drink, change it regularly throughout the day. If you are going out, take a portable bowl and put cool water in a thermos flask or cool bag to keep cool. Know the signs of dehydration and react fast.
Get a paddling pool and put a few inches of water in it or fill it with sand and wet the sand frequently, they can dig in the sand and lay in cool sand or lay down a wet towel for them to lay on (if necessary with a plastic mat or bin bag under to stop the water seeping through).
Avoid hot surfaces like asphalt, tarmac and even sand. Touch it, if it is too hot for you to walk on it will be too hot for them. They can get burned paws. If you need to walk on it, try and find areas
that are shaded to walk on or buy them dog boots, rubber paw pads or a paw protector wax.
Do not over exercise them in hot weather and avoid the midday sun (11am to 3pm) Start enjoying early morning and evening walks, its light so enjoy it. If you need to go out in the midday sun, go somewhere they can swim or paddle or is in lots of shade.
Get gel filled cool aids. A cooling collar, cooling mat and cooling vest with built in gel filling that goes in the fridge or freezer until it is needed.
Dogs can burn in the sun. If they have pale skin, put them on nontoxic sun cream and even a t-shirt to protect them.
Shade is very important. If they live outside, ensure they have access to shade at all times. If travelling in a car, ensure there are sunscreens, if you would sunscreen a baby, sunscreen your fur baby. Do not ever leave them in a car. Space blankets can be used to cover their kennel or crate to give them some well needed shaded areas.
Keep the sun out. Keep curtains and blinds shut when the sun will be coming in through them and ideally have a blackout blind to stop the sun coming in the house. If you do not have air conditioning, have a fan running for them.
Use a mesh elevated dog bed to help air circulate and keep them cooler.