A study conducted by MarketingSherpa in 2007 concluded that subject lines that contained 'action' words "attracted the eye and elicited opens" ⁽¹⁾.  Action words call on the recipient to do something such as Confirm a registration or Verify their travel itinerary.

Here are some examples:

• Please confirm your BombBomb trial account.
• Here's $20 off, thanks for shopping with us.
• Reserve your seat for annual gala.

The top three types of subject that compel subscribers to open emails contain one of these⁽²⁾:

• Discount Offer
• Free Product Offer
• Familiar Brand Name

The top five birthday subject lines based on open rates are⁽³⁾:

• A special gift for your birthday
• Happy Birthday [FIRSTNAME]
• Happy Birthday From [Company]
• Your Special Birthday Bonus
• A special gift of 20% off for your birthday

For more ideas about the Subject line see Improving Opens with Subject Line Length

⁽¹⁾MarketingSherpa Email Marketing Benchmark Guide 2009

⁽²⁾MarketingSherpa "2010 Email Marketing Benchmark Report" (2010)

⁽³⁾Experian Marketing Services "The Birthday and Anniversary Report" (2010)