Are your guinea's inside or outside, if they're inside are they in a room you are always in or their own room - mine have their own room and I keep the radiator off all year round and put fleecy blankets over them on cold days/nights and leave the door open... The rest of my house is pretty hot.
There are a few good tips on "http://www.theguineapigforum.co.uk/showthread.php?t=48840&highlight=cool+summer" this thread.
hope that this helped a little and I'm sure a more experienced member will be replying soon
My inlaws piggies in New Zealand had a swimming pool - not a big one - that they could reach when they wanted to with a ramp. So they would go for a dip to cool down and chill out. It worked and they enjoyed it. The pool was a few feet deep, they couldn't touch the bottom, but they still liked it. It rarely gets hot enough in England to warrant this!
how about freezing a big bottle of water and then wrapping it in a towel and putting in George's cage for him to lie against or near when he gets too hot, i've done this with my boys.
I also made ice cubes with chunks of cucumber inside, so they could lick them
Thanks so much everyone! I'm gonna try the freeze food in an ice cube one and the bottle one and the swimming pool one and much more. Man George is gonna stay cool lol. And to the first reply I keep George outside in a hutch.