That's the one area a lot of people overlook, underwear. If you're using cotton, it absorbs moisture and you get a sore rear. When I switched to using synthetic shorts it made a world of difference on long rides. I had a friend with a Guzzi who complained about monkey butt syndrome on long rides and was considering a new seat. I told him before he changes his seat he should change his underwear (to synthetics). He was astounded at the difference and is now happy with his stock seat. Below is a company which makes motorcycle specific shorts, but I found that the Merino brand synthetic version at Target worked well for me.
Moisture - Tri Monte Group Inc DBA LDComfort