The primary issue with seat comfort as far as I can tell is how much surface area it provides. The more area you are able to spread your weight across, the less pressure being pinpointed in any one area of your body.
Adding sheepskin, airhawks, new foam on the same saddle will only provide a minimal improvement. Perhaps buy you an extra hour or whatever before you experience discomfort.
I view this as more true the heavier the rider is. Those who weigh a lot less are probably far less likely to experience hot spots or discomfort as soon as someone 100lbs heavier. In those cases, better foam or airhawks probably have a higher chance of working out well. For me, at 260 lbs, they will never provide a great improvement. I need the greater surface area something like a Russell provides. Hot spots/discomfort are inevitable but I think the Russell will allow for the greatest distance before it happens... Probably by a large margin. Wish the Explorer community had a "floating" saddle for this purpose!
I have found that highway pegs, used for a few minutes every hour so, work quite well as it alleviates some weight from the saddle a little bit. They aren't as comfortable as the standard riding position so I use them sparingly but they do seem to have a positive affect.