Other features, like full bench construction including blending the back-cut into the backslope, will allow water to roll off the trail properly. On existing trails that have chronic wetness problems, look up the hill or trail. Most problems start uphill. As the water gains momentum, and gathers more force, the more destructive it can be. Trying to re-direct where it begins can allow a problematic mucky trail to be much drier.
Spring is a good time to assess the water and wetness associated with a trail.
It's even better to scope out the trail while its raining, so that you can see exactly how and where the water goes. Then in the fall, go back and do the work to re-direct the water. This allows the trail to settle in during the winter. We were all introduced to Pulaskis and McLeods. A Pulaski is a combination of a hoe and an axe while the McLeod is a rake-ish sort of tool.