I can tell you what NOT to take! In Dawson Creek we encountered an older couple in a truck camper. It had been raining and there was mud everywhere. That poor lady was dressed in white summer clothes. I'm sure she was chilly and she was trying desperately to keep her light colored clothes clean. What you need are jeans, turtlenecks, flannel shirts, warm windbreaker jacket, warm cap, and even gloves in some places. If you go out on the water such as at Prince William Sound you'll want to dress warmly. You probably won't wear much in the way of shorts and sandals, although Fairbanks was around 90 degrees when we visited so you'll want some clothes for warmer weather but concentrate on cooler weather clothing. Rain gear is essential. You want clothes that will cover your arms and legs to keep off the mosquitoes. Seriously, if you take the bus tour into Denali have a hat with netting to keep them off your face. I got ours at an army surplus store. In addition to bug spray take something like Benedryl in case you do get bitten and it's driving you nuts.
Now don't let all this turn you off because going to Alaska is a wonderful experience. We've been twice and loved it both times. Wonder Lake at Denali is beautiful. The critters are fabulous. Taking a river boat cruise in Fairbanks is fun and educational. Most towns along the Alcan Highway have museums and they're all different. I enjoyed reading a sourdough's diary about getting to the goldfields. The Yukon Beringia Interpretive Centre in Whitehorse is fascinating; it's about when the land bridge existed between Asia and North America. Oh yes, if you like fish you'll be in heaven with the fresh salmon, halibut, and shrimp.
I hope you have a good amount of time for your trip because there's a lot to see and do all the way up and back. BC had/has? all kinds of factory tours related to the lumber industry. One made Oriented Strand Board. Another was Home Depot's biggest supplier of finished lumber. We took a month just going from the San Francisco area to Alaska and then spent another six weeks within Alaska. Hope you have a wonderful trip!
ArdraF