Make sure your dog has access to clean water at all times, ideally a large bowl filled to the brim. Carry water and a bowl on your walks.
On hot days, walk your dog during the cooler parts of the day, in the early morning and late evening.
Watch your pet for signs of over-heating, including heavy panting and loss of energy. If you recognise these signs when on a walk, stop, find a shady spot and give your dog water.
Never leave your dog alone in the car, even with the windows open.
Make cooling tasty treats by making ice cubes with your dog's favourite food inside and pop it in the freezer.
Be particularly careful with short nosed dogs such as bull breeds, boxers, pugs, older dogs and those that are overweight. Those dogs can get heatstroke by just running around!
Dogs paw pads can burn on hot pavements. As a general rule, if it's too hot for your hand on the pavement it's too hot for their paws. Dog booties can be bought from pet shops and online, or walk at cooler times of the day.
If it's too hot for the usual long walk, keep your dog mentally stimulated by doing some refresher basic training with some sits and stays, or teach them new tricks. Use positive reinforcement training by praising and rewarding the behaviours you like and ignore the behaviours you do not like. You can use praise with life rewards (such as games, walks), or treats, clickers to reward your dog's good behaviour. Treats should be taken out of your dog's normal daily food allowance/calorie intake. (Treats should be made up of no more than 10% of your dog's daily calories!) There are plenty of low calorie treat options.