Been an old school marketeer of posting ASCII/text formatted job opportunities to targeted, opt-in candidates. Now that I'm using Constant Contact since May, am seeking out other Recruiting/Sourcing Experts who have had relative success with their campaigns and would like to share strategies, timing, and templates. My open rate generally averages 35-40%, and would like to push it up over 50%. My niche focus is information technology professionals who possess active high-level security clearances and work for various companies that have contracts with the Intelligence Community and Department of Defense.
Congratulations on your current open rates! It's always admirable to strive to do better, but I must say that fighting for a 50% open rate average is a pretty rough struggle. A number of your contacts may be viewing your emails in preview mode, or on systems that block our tracking codes. It's also likely that some of them may just not be available for a period of time, whether they're on vacation, or just too busy to check all of their emails. Considering how specific your target audience is, I'm quite impressed with reaching a 40% open rate. Most professional services like job postings and job-search assistance have open rates of closer to about 16%, under half of yours.
Some of our current best practices can be found here. I imagine that you're already following a large majority of those, but it might give you some ideas on the most impact-full aspects of marketing, to help you narrow down what you want to perfect.