Allow the sun to shine on the muddy ground. Obviously the sun will dry up water, but is the area of ground you are trying to dry out getting its full potential of sun exposure? If there are tree limbs inhibiting the full sun from shining on the spot of ground, consider removing them.
Use lime in the forms of quicklime or hydrated lime. Lime will quickly absorb moisture from clay and soil. Spread the lime over the affected area, mix it into the earth and pack it down with a shovel.
Spread gravel over the muddy ground. While this will not absorb moisture, it will create a firmer foundation if your goal for drying out mud is to build something on it. This may be the last resort before contacting a professional landscape architect.
Hire a landscape architect to professionally analyze your mud problem if other methods do not help.