There's nothing like persistence to get a job done - especially when most people give up after just one or two efforts.
It's particularly important in marketing for 2 reasons:
1) When there's such a noise coming from advertising in general, it often takes more than one appearance to get noticed
2) If you keep showing up, you'll be front of mind when the customer decides it's the right time to buy. (For an excellent take on the importance of timing and repetition, see Seth Godin's blog entry on this subject.)
Newsletters do the repetition thing very well, of course. They keep showing up in your prospect's or client's mailbox - and on top of that are an entertaining and useful read (if you do it right).
So don't give up after just a few editions. Keep showing up and you'll be at the right place...at the right time. (Betcha your competitors don't do this, either!)