Maxi-dresses are a must for summer pregnancies. I had DS in August 09 and this one is due early September despite my vow of no more summer babies!
A pedestal fan is a life-saver, on really hot days I used to just strip to my undies (remember to close the blinds!) and lay on the sofa with the fan blowing.
Get a water bottle, fill it every night with your drink of choice, and put it in the freezer (don't fill it full though or it'll crack, leave an inch or two at the top). Next day take it with you wherever you're going, it will defrost slowly so when you get too hot you've got ice water. This is brilliant for labour too.
Cotton underwear is a must have. You'll thank me when it's 80C outside.
Poundlabd sell little fans with a water bottle attached, spritz while it's spinning and you have water mist. You can also get spray cans of cold water mist from Boots, they're a little pricey but one can last ages and they're often on 3 for 2 or 2 for £x-amount. Also great for in labour.
Get a parasol for the garden so you're not in direct sunlight. On very hot days, if you're able to then go to the beach because it's usually cooler on the coast.
Linen and cheese-cloth type fabrics are nice and cool.
Keep one of those gel eye masks in the fridge, brilliant for cooling you down on a hot afternoon.
Keep the freezer loaded with ice pops. If it gets too hot to sleep at night then pop your pillow in a carrier bag and bung it in the freezer for an hour before bed (if your freezer is big enough)
People will point at the sun and then at your belly and then say stupid things like "I bet you're hot today" with a stupid grin on their faces. Resist the urge to punch them.