This may seem like overkill, but anyone who's ever had his luggage lost will attest to how handy a packing list is. It's pretty simple. Just make a list of every single thing that you pack -- your clothes, valuables, toiletries, medications, everything. If anything happens to your bags, you'll be glad you have a detailed list to file a claim with the airline or cruise line that's responsible. It's the only chance you have at getting the full value of everything they lost. As far as the smaller items like toiletries, this will help you remember to replace them. Otherwise, if your bags get lost you'll spend the rest of the week remembering what small items you're missing as you need them. Put the packing list in your carry-on travel pack.
If you think a packing list is unnecessary or too Type A, just remember this article when you're staring blankly at your luggage claim form, only able to recall bathing suit and underwear.