I think what the road building experts would say is, "Drainage, drainage, drainage."  If you can get the water away from the road, the mud won't be so much of a problem.  

If there is a spot where water is trapped on one side of the road and needs to drain off to the other, but it's not an actual stream that needs a culvert installed, often a "rock cannoli," or a series of them at intervals, can make a big difference.   You dig a ditch across the road s you would for installing a culvert, line the ditch with a big piece of Geotextile fabric, fill it with rocks, (any old local rocks will do,) then fold the Geotextile over the top and cover the whole thing with gravel.

The experts would also probably recommend laying down Geotextile fabric in problem areas, then putting in a substantial layer of gravel on top of that.  If you don't have the money to do that to the whole road all at once,  either do the worst spots first, or start at the beginning of the road and do another section each year until the whole road remains passable.  (By then it's probably time to start at the beginning again, since a road can be expected to lose a little gravel each year.)

There are links on the Resources page of this website to some excellent manuals on gravel road maintenance.  Also Maine ROADWays' website has a page on low cost road maintenance tips.  Here's a link:

http://maineroadways.blogspot.com/p/low-cost-road-maintenance-tip-of-day-if.html

It includes things like arranging for fuel deliveries before the frost goes out, making trips out and back early in the morning while the road is still frozen whenever possible, and avoiding driving in the same two ruts.