Not to get nasty here but er.. try some vasaline. Ok, all jokes on that aside, it does help seriously cut down on the chaffage. Just use two fingers and scoop a little in to the undergarments. Then apply a little to the trig before putting on the pants. Get the smallest vasaline bottle/tube you can find (50g is what I have for my chapped lips) so whenyou have to go to the loo to do you business you can reapply a little vasaline to the trig before you set off to ride again. Err.. it's best to use the enclosed loo when reapplying the vasaline else you'd be gettng a lot of odd stares.
Hope that helps. BTW I've never done this before however I've had someone that used this method and they has relief.
Zero_Enigma