Cross-posting stands in stark contrast to Multi-posting (whereby the same material is posted several places--without any links between the occurrences and without all information, in the form of commonly-shared responses, available to all who read the original post).
Reasons for cross-posting (as opposed to posting the same item individually to several groups):
How to cross-post:
1. Enter each of the groups to whom the post should be submitted on the Newsgroups line (or To line); separate the groups with commas.
2. NOTE: Some people find the following technique nearly as objectionable as Multi-posting, in that (if you are not reading the post in the group on the Followup-to line) in order to read the responses, you must leave the group you are in.
Select a single group to which you would like the responses to go and put it on the Followup-to line.
If you use this method, do note in the body of the post that Followup-to is set. (The worst aspect of the Followup-to technique is that it is so often done improperly.)
As always, make sure that the Subject line accurately describes the content of the post.