1. Provide shade
Wherever possible, move all potted plants into shaded areas of your garden. If there isn't enough shade, you might use shade netting to keep your plants cool.
2. Water more generously...
...and most importantly, do so at the right time of the day. Avoid allowing moisture to get too hot by watering your plants either very early in the morning, or - even better - after sunset.
3. When you get new outdoor plants delivered...
Firstly, take them out of the box and put them in the shade as soon as possible - that applies to the sun-loving ones too. They need some time to adjust to their new surrounding, and placing them in the scorching sun with no introductory period might really harm them.
Water your new plant generously as soon as you take it out of the box - it might appear a little more thirsty when it first arrives, but that is absolutely normal, just give it some time.
Water twice daily, if needed, and don't be alarmed if some appear to wilt in the heat of the day - most will recover once the temperature falls again in the evening.
High temperatures make moisture evaporate much faster, a layer of bark or moist compost will help keep it in, and protect your plants.