Dr. John Sullivan has written many articles on this topic over on ERE. Check out his list of articles there for a wealth of great ideas.
My two cents:
- Pick a theme that will resonate with the culture of your organization and get their attention. Avoid using monetary rewards as the core of your message.
- For rewards, select those that will incentivize the behaviors that you want to influence. If you need a greater quantity of referrals, incentivize at the front-end of the process, if you want greater quality, incentivize referrals that result in interviews. You can offer low-cost incentives at this stage that can be just as if not more effective than the big-dollar incentives that often result when a hire is made.
FYI - Michelle Hinkle of Booz Allen and myself will be participating in a HCI Talent Scouting Magazine Live Interview tomorrow (Thursday April 17) at 1-2PM ET. The subject appropriately will be Employee Referral Programs.