How far ahead you plan your Alaskan cruise really depends on what you're looking for. Alaska is behind only Europe and the Caribbean in cruise destinations worldwide, and since the season is short, cruises can fill up quickly. If you're looking for a certain type of stateroom, you'll want to book as early as the cruise line will allow, several months to a year in advance if possible. If what you're looking for is a bargain, you might want to consider holding off and seeing what's available at the last minute. Empty cabins don't make money, so if there is space available, you might find a really good deal.
Another thing you need to decide when planning an Alaskan cruise is what type of ship you want to take. The larger cruise ships offer more comforts and amenities, but the smaller, local ships can make it into tight passages more easily, allowing for more scenery. You'll also need to decide if you're going to take a one-way glacier cruise, or a round-trip Inside Passage cruise.
Another important part of planning your Alaskan cruise is deciding what to pack. Remember that many cruise lines have restrictions on the amount of luggage you're allowed to bring with you. The nights in Alaska can be cool even in the summer, so you'll probably want to dress in layers. Check with your cruise line before you leave to find out if you'll need formal wear for dinner. Make sure you have a jacket and a raincoat. Bring a pair of sturdy walking shoes for shore excursions. You'll want to have binoculars for whale watching, and don't forget your camera. Most cruise ships have swimming pools so bring your bathing suit, and don't forget the sunscreen.
Check with your cruise line to find out what is offered for shore excursions before you leave home. Most lines offer many different choices, from whale watching to hiking. One thing is almost certain, you'll have a chance to see many different types of wildlife and breathtaking scenery.