I really am very interested in newsletter marketing, and want to begin
campaign on several niched sites I run. Of course I'm taking about opt-ins,
not unsolicited mailings.

For each niched site I'll offer newsletters, I'm going to offer tips/suggestion box,
main (or several "mini") content(s).

So besides the frequency of the newsletters, and the layout of them (either
plain text or HTML) - what techniques do you all use in your newsletter marketing?