PDN has an ongoing series called "Promos I Kept" where they interview creatives about what self-promo pieces are effective. The most recent installment has some really great and solid advice from JWT Director of Art Buying Shawn Smith. To read the whole piece you must have a subscriber log in. If you don't already have a subscription to PDN, you need one. On email promos:

Shawn Smith: I get tons of them, at least 50 a day, especially on Tuesdays, Wednesdays and Thursdays—it’s an insane amount. The problem is that I open and read 90 percent of my e-mails on a Blackberry so if you are sending me images in an e-mail, chances are I won’t see them.
The best e-mails are very focused, event-driven announcements about a show or a new book that’s coming, things of that nature. Keep in mind that creatives are not constantly looking for photographers, we’re doing our other job too.

On personal projects:

I think it’s really important for photographers to be working on series and stories and sequences, rather than just single images.

On print promos:

I really like when people send photo series, show announcements, book announcements, etc. I also love to receive small books, though I do understand that cost can become a big issue when doing these types of piece.

Interesting bit about all of the email promos coming in on Tuesdays, Wednesdays and Thursdays. I advise photographers to send emails on Tuesday or Wednesday, because the data consistently shows that those get the better open rates. But if everyone is sending on those days, it will become overwhelming for the client to even begin to look through all those emails.

Best advice: only send out an epromo when you actually have something to say. Don't just send one when you have a new picture to show off. If there is something interesting or newsworthy in your promo, you have a much better chance of the buyer reading it and clicking on a link.