On the road, I always clip out on the left which seems to have solved it for me.
Too much lube can definitely contribute to the problem, but mtbiking you are going to pick up dirt no matter what. Ace was asking me the same question this weekend so I was thinking about it. I don't get much grease on my legs even mtbiking when you have to clip out on either side. I wonder if I may also have trained myself to put my foot down a bit farther out to the side. This accomplishes two things - less grease and more stability on uneven trail.
I'm much more likely to have dirt on the tops of my knees from sitting on the top tube during breaks and brushing against the tire.
Frends know gud humors when dey is hear it. ~ Da Crockydiles of ZZE.