Two easy things I did to erase this issue for me:
Got some biker short undies.
Put a swipe of antiperspirant on the spots I know will be trouble before I hit the trail.
The first helped me stay much drier in general and eliminated any rubbing from seams. The second prevented the salt from sweat abrading those tender spots. Also, the skin coated with the stuff tends to just slide a little better.
Cost was under $15 total.