Let's forget about deliverability for a moment and focus on the intended target. How are you going to get them to open your email? Email marketing seems to be all about deliverability these days, yet many are forgetting a very important aspect of their campaign; getting their recipients to open the email.
What's the point of putting together an email marketing campaign if you can't get people to open your emails? After all, that is how they are going to click through to your site. Of course you want to get the best deliverability and reach the inbox with every email. But let's face it, you have to have a killer subject line to get the unique opens. Even before you start to think about subject line spam filters, get an idea of what you want to say.
HAVE SOME FUN!
If your business is in a super serious industry, stay serious. Otherwise, have some fun with the subject line. Stand out from the masses. Be memorable and entertaining. Stay away from monotony; it is a killer.
Clients do not want to be just clients anymore. They want the places they give their business to to be personable and provide superior customer service. They expect this even in the emails they receive. If your company deals with a product used primarily in the morning, ask "How was your coffee this morning?". If you are selling a travel deal, try something like "Don't forget your carry-on" or "Palm trees, sunshine, and you".
After you have come up with something crafty, let's make sure it is legit and legal. Remember that CAN-SPAM Act? Make sure your subject line is relevant to the body of your email. Also take a look at some spam avoidance guidelines to make sure those pesky spam keys are not in your subject line. This will help ensure your deliverability.
Make your campaign all it can be! Go out, email, and have some fun.