A rule of thumb is that your planning should take as long as you're building. Focused water does the most damage, therefore, keep in mind how to plan for the water to be 'broad' and unfocused. This involves things like waterbars to direct the flow of the water. 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.