There are at least two possible sources of sore bum. On a long ride they'll double-team you.
First is reduced blood circulation due to compression of the blood vessels, leading to numbness, and then pain, as the GS riding stance puts most of your bodyweight straight down onto your arse. Being fit and supple helps a lot, as your circulation is in better condition to start with. You can also take a bit more weight through your thighs for 10-20 seconds every so often, which takes the pressure off your cheeks and restores circulation. I've even resorted to slouching with one cheek alternately off the seat, at which stage I know it's time to take 5 mins break and get full circulation restored.
Some GSers have used the trick of setting the seat height adjusters differently front and back. If you drop the front lower than the back, you'll be leaning forward slightly and may be able to take a bit more weight with your shoulders and arms. Or, you may just crush your balls - but it could be worth a try.
The second cause is skin irritation caused by sweaty knicks chafing, which is obviously more of a factor in hot weather. You can reduce this by keeping your body temperature comfortable: strip inner layers, open vents in jacket, and so on. You can also swap regular keks for cycle shorts, which don't have seams and don't ruck up when you shift around.
The GS is designed to be ridden standing in the pegs, which gives a nice cooling breeze 'twixt the gusset in hot weather - ahhhhhhh... And as always in hot, sticky conditions, drink plenty of water.
As any keen cyclist can tell you, there's also the factor of butt-hardening - somehow your bottom gets tougher just by regular conditioning.
Probably a bit more intimate detail than necessary, but HTH helps anyway
