Dry.
My manual does not mention wetting anything.
(The stock one in my LIKE200iLX was not wet)
Just says replace with a new one if too dirty.
Wash, dry and re-use the foam layer.
Stig
* and yeah, the airfilter in a LIKE200i stays clean a very long time. Must have something to do with the clean air space where the end of the intake snorkel is positioned.