I'd say the best source of ideas on more effective referral programs are to ask your referral sources. Even surveying them with a very simple survey could not only provide some added insights but also reinforce your gratitude for their continued loyalty.

What you might be under-estimating is how good the IT Service firms referring you look when they're able to get their clients a great web development outfit on time and on budget, or at least, I presume this to be the case ;) There are so many bad web shops out there, that just having a reliable referral source might be value enough.

Also, if referrals are being made by only a handful of employees inside these IT service firms, you might consider just spending the fee on a great regular dinner or lunch. High-touch relationship building is often more effective as a lubricant to further business amongst service providers than elaborate referral / incentive programs.

Happy to talk to you further about how to evaluate your options.