Watch the weather forecasts and be prepared. I have heard that Alaska has been warmer than usual this summer.
We sailed from SF three years ago on Sept 19. It was the last cruise of the year. My advice is to make sure you have one warm parka, a hat and gloves. At Skagway we ran into the first snowfall of the season and it was cold for us wimpy Californians. Some actors did an outdoor performance (it was cold and rainy) and even with all of the above I gave up partway and went inside to sit next to the pot-bellied stove. We were told that the cruises a couple of weeks earlier had up to 80° weather and sunshine. We couldn't do the inside passage because a storm came up.
Most people were dressed more casually than usual on a warm-destination cruise. I'll go back and look at my photos and see what else I can think of.
Victoria was very nice though.