Good saddle
Good shorts/bib shorts...flat seam padding
Liberal use of Chamois creme
Time on bike...how long is your usual ride?...how often do you ride a century?
Good fit on bike
Change position more frequently

Dealing with a saddle sore:
Use a product such as "Boil Ease" to reduce swelling
Use of a soothing product after the ride/before bed/etc. to soothe such as A&D Ointment, Noxzema, etc.