My last "long" ride was 9,000 miles in 12 days, including "short" first and last days of around 400-500 miles, and one full day off for new tires and general bike maintenance. Oh, and a thousand-odd miles of gravel in the middle.
As others have said, a custom seat built to your needs is the best solution for multiple long days in the saddle. These can be pricey, but they do work very well.
A seat cover is the next best—Airhawk, gel pad, sheepskin, BeadRider, etc. These allow you to swap out between using one and using nothing at each gas stop, which can help to change things up. However, with a properly fitted custom seat, you don't need to put anything on top of it.
A good backrest can also be quite comfortable for extended day rides, and helps you to retain good posture which keeps the back happy.
But one thing often overlooked is what you wear between you and the seat.
Jeans or work pants are good for everyday wear, but just don't cut it for long rides. Besides having seams that you sit on which cause irritation, they can retain moisture (either sweat or rain) which is bad for your skin. And they offer no padding or abrasion resistance, so even a very low-speed slide will result in some serious damage to your legs.
Personally, I like the
Aerostitch AD1 Pants, which are waterproof, breathable, comfortable, and have no seams to sit on. Under that goes
LDComfort Undergarments, which are also breathable, comfortable, devoid of seams to sit on, and actively wick moisture away from your skin to avoid sweating, rashes, and irritation. I've tried many other brands of under- and over-pants in the past, and none of them have worked as well as my Aerostitch and LDComfort gear.
A good seat can really add to your long-distance comfort, but what goes between you and the seat matters just as much.
