Ryan,
I also work with MM enamels and dry brushing can be a bit trying...to say the least.
Here are some of my general techniques:
1) When you think that all the paint is off the brush, it's not. Granted, there's not much there, but you don't want very much at all on the brush. Trust me...when you think you've rubbed allt he paint out of the brush, run it across a few rivets or bolt heads and across the edge where two panels meet and paint will come off.
2) I use 3 or 4 very soft sable brushes between 1/8" and 1/2" wide when I dry brush.
3) In my experience, you can only apply paint to each brush 3 or 4 times (and brush off the extra paint on a paper towel) in a sitting before you will either have to clean the brush and let it dry or switch brushes. This is because the enamel paint with start to get "gummy" and will bind the bristles together or start to clump up.
4) Each time you apply paint to the brush and remove the excess, you only have about a minute to maybe a minute and a half to work.
5) Take your time. I will typically spend 2 or 3 sittings for a simple dry brushing project and up to 15 sittings for more complex projects. I've found that the slower I go - applying the absolute minimum paint in each sitting - the better the dry brushing looks.
6) No one masters dry brushing...they just get pretty good at it with years and years of practice. This is my personal belief and it works for me.
Hope some of this helps,
Enjoy your modeling...