You might want to try something like
this.
They're made from the same material some sport apparel is made from and wicks moisture away from your head/skin. These actually work pretty well in the summer heat, but depending on the amount of time in the helmet, can get the liner wetter than without it. The idea is to get air flowing around the head and promote evaporation on the skin, which cools you. Helmet liners can be washed and should be dried out as quickly as possible to quell bacteria growth.
What kind of helmet do you have? I just picked up a KBC and it has incredible airflow across the top of my head. It's a little noisy with wind, but that can be alleviated with ear plugs or headphones. The liner completely removes and is machine washable.