Just pack clothes that you can wear in layers and peel off as the day warms up, and of course some kind of rain gear, gloves and hat, especially if you are visiting a glacier, they tend to be cold up there. Here is a link to the CM Alaska 2003 planning guide, lots of great info and tips:
Be sure to take plenty of film as you will be amazed at the beauty, but be sure to pack them in your carry on, the x-rays that they will use on your checked luggage will ruin them. We went the first week in Sept. last year and had a few afternoons in the 70's, so I did wear shorts on those days, so may-be pack a pair too?
Have fun, would love to read a review and see some of your pics :-)