After struggling for a long time see thread here
trying to find a plug-in that will allow a user to export to STL directly from within Sketchup AND ignore lots of errors at the same time. I have found this one: STL4SU
This seems to have been created in April 2010.
I have just recently tried this and it worked perfectly on a model that I knew to have errors and was giving me all kinds of trouble with other methods (except CadSpan that is still a good option).
(Edit: CadSpan has a triangle limit. My latest model has 13000+ triangles and they won't accept it).