Good cornering technique is never more important that in slipper mud.  Make sure you always corner by leaning the bike over, not just turning the bars.  This gives a smoother turn lets the grippy side-knobs of the tyre get into the trail.  As they bite through the mud they stand a better chance of stopping the wheels from slipping out beneath you.

Tracy Moseley: Mountain Biking in the Winter in the UK

At the same time as leaning the bike be sure you drop your outside foot, and commit as much weight to it as possible.  This puts your weight at the lowest point possible, and almost directly over the contact point between tyres and trail, encouraging extra grip rather than sideways slip.