The trick is to get FULL length neoprene sleeves, that go from the upper tube all the way down to the bottom of the chrome lower tube. If they are securely mounted then dirt or mud will simply not get in. You just have to make sure that they do not limit fork travel by bunching up too much at full compression.
Colebatch has over 60.000 kms, most of it off road and with plenty of mud, on the same set of fork seals on his USD forks. Others who rode the same tracks without sleeves had leaking seals many times, requiring replacements often.
For seal cleaning (not always a solution), the Risk Racing Seal Doctor is great though.