In Marketing Sherpa's Email Marketing Benchmark Report for 2012 (note the free excerpt offer!), email marketers reported an average clickthrough rate of 26%.

There are many long-term endeavors marketers use to increase interest and trust, and in a roundabout fashion, clicks: things like responding to all email inquiries, being as transparent as possible and sending on a regular schedule. But there are also ways you can attract clicks with the email itself.