What's best handling in mud, a skinnier or wider tire?
Derek,
I love the mud, I like a wider rear tire for the mud, but not too wide.
Also don't run too much air in your tires, try 12-14 lbs.(max)
If you are a slower rider you might even be able to go down to 10 with out worrying about flats.
The XR 400's are fine bikes for the mud.
How are you approaching the mud?
Are you standing or sitting? You need to be standing, with your weight on the pegs it lowers the bikes center of gravity and will make it easier to navigate the mud.
You also want to hit the mud with some momentum and try to keep that momentum up with out spinning the rear wheel any more than necassary.
Also line choice makes a big difference. You can often stay out of the ruts and do better than trying to ride them out, sometimes there is no other lines but try being creative. I look for vegatation and try to stay close to it if possible, just one good clump of grass can give you a little extra traction to help make it through. Also if the mud hole is next to a tree, that means there is usually some roots. Roots can throw you down with out any notice.
IF all else fails, just buy a CR 500 put on a paddle tire and just gas it, oh yeah hang on tight.