Provide fresh water, maybe a small pool, a shady area, and a place where they can dig themselves in. Dogs will typically dig up an area ,usually under bushes or in small shaded area, where they'll spend most of the heat of the day in. They remove the warm topsoil from the earth to expose the cooler earth beneath, which in turn keeps their bellies cool. If they don't already do this you might consider doing a small area for them where it's not an eye sore, assuming their outside. Indoors all you can do is provide a constant supply of water, keep the house temperature cool, and if you have tiled floor try to keep it exposed so they have something cool to lay on.

I  have one additional suggestion. I go to Walmart and purchase one of those cheap plastic baby pools and keep that filled with cold water for my 2 dogs. They can get in and cool off when they want to. If your dog likes water but has never got in a little pool, just step in and show them it's nice and they will follow suit.

Make sure you keep giving them a fresh, cool bowl of water, emptying the bowl and refilling it with fresh will be better for them, also make sure they have somewhere cool to rest and try not to walk them too much.

Make sure they always have a cool place to seek refuge: Shade under a tree or, if inside, a basement or room that is in a cooler (North) part of the house.  Keep plenty of fresh water available to them at all times.  A cool bath or run through the sprinkler or hose is refreshing as well.  Don't allow excessive exercise midday as this can cause most dogs to overheat if the weather is hot.