If you've done any longer trips in the lower 48 you already know everything you need to know. Alaska's a long trip, yes, but it's not qualitatively really different from riding in the western half of the USA. Just more days strung together.
That said, some thoughts:
Unless Prudhoe Bay is a big bucket list thing for you, consider spending that time elsewhere.
Related to the above, don't miss the Kenai Peninsula.
Also don't miss the Icefields Parkway between Banff and Jasper.
Tools and spares, pretty much the obvious stuff. I like to have levers, brake/clutch/shift, so that if I break one in a drop or minor crash I'm not stranded. I make sure that I can get either wheel off the bike, can plug a flat and pump the tire back up, and can get the tank off to poke around if needed. Some zip ties and electrical and duct tape too.
Rain gear. Really good, no-shit, ride all day in rain and stay dry rain gear. That includes boots and gloves that work all day in a cold, miserable, driving rain.*
Be prepared for big variations in temp over the course of the trip. If you haven't done much cold weather riding before, make sure you do some before the trip so that you know your gear is up to the task. Include long, all day rides in that test.
Most of all, just relax and enjoy. The roads are good (although there will be some construction) the towns aren't all that far apart and the scenery is amazing.
*If you read a lot of Alaska trip reports you'll quickly figure out that rain is the dominant topic. I was actually very lucky, only had 2-3 days with significant rain, but some friends who did the exact same trip the next year had rain almost every day. Be ready for it, otherwise you'll be miserable.
'07 DL650, '01 DR-Z400S, '99 SV650 (race bike), '80 GS1000S, '85 RZ350, '08 Ducati 848