Attribute the source of your joke. Tell me anything you know. If it's one line from a comedian's routine, name the comedian. If you heard it from your brother, say that. If you wrote it yourself, definitely say so, as I will give it preferential treatment.
Excerpts are OK, if they take the form of news, or a review of the work.
Don't send me professional columns like Dave Barry's. I just can't send these out without permission. Instead, if you see a posting in a non-humour group that you think would still be very funny when taken completely ot of context, mail the poster and encourage him or her to submit it to rec.humor.funny. Mention the RHF policy sheet to them when you send this note of encouragement. It's amazing how bad the submissions I get are in this department.