This is one of the best summaries on good email marketing I've read in ages. Two year's worth of research broken down into four actionable areas.
Deal: Percent off often performs better than dollars off.
Subject: Six to Ten words and punctuation, like question marks, help.
Timing: Open and conversion rate peaks differ by month; summer is good.
Body: Multiple calls-to-action and multiple products--that can lay out cleanly.
All good points to consider.