How do i keep them cool?

In the warm summer months i find it hard to keep them cool and ofthen find the guineapigs lying down in the shade of their cover but do you have any sujestions on how to keep them from getting heat stroke?
Please help.
From Lauren Gillanders

u can get frozen water bottles ect from the freezer and put them under a towel for a guinea pig to lean on..and just if their outside in a cage..put a cool towel in the cage..hope i helped.