A little while back I posted some info about email promos and I made the offer for photographers and illustrators to send me their epromos for some feedback. I had a few people take me up on the offer (which still stands for anyone interested). Armando Bellmas had a great question: "What kind of subject lines make you open an epromo?"
It's always a little tough to say because everyone has a different reaction to the subject line. I've run across subject lines that were witty, some that were cheesy, and some that reflect the work. I try to open all the email promos I get.
I always like the "New Work by..." or "Hi Caitlin" - using someone's name and making it personal is always a good way to go... although I'm sure it would be hours of work for for the sender.
There's no sure-fire way but the subject line is usually what makes or breaks an email promo to be opened. You have approximately 2 seconds to grab your viewer's attention with the subject line and take the action you want (which is ultimately to go to your website). The other tricky part is to keep it away from junk file or spam detector software.
A helpful hint is to check out newspapers and magazines. Writers have to sum up the article in about 5 words to get the audience to read the entire piece. Making an announcement or sharing news is a great way to get the email opened. For example: Photographer won creativity award or Photographer just shot X campaign.
I would suggest testing out a few different ways and see if you get any feedback and what works best with your target audience (which subject line delivered the highest click through rate).
My "edit, copy and proof" college professor always use to say "when in doubt KISS"... Keep it simple short (or "keep it simple stupid" he preferred).