If you go to any good quality bike clothing store they will usually do a 'loose fit' jersey.

It is usually made of good material which will draw the moisture away from you body. I think Nike do a 'clima-lite' range of sports clothing which draws moisture out.

Also there is a great range of jerseys from top brand Endura which are 'loose-fot' and are supposed to go over padding but I sometimes find them much more comfortable.

Its a bit trial and error with these but once you have found one you like, you will be sorted forever!