ways to invite sororites to mixers?

[pythagoras]

I was wondering if anyone could help me by sharing how they went about inviting sororities to mixers, and/or what way was the most effective. We have a great reputation on campus, but we're relatively new, so we don't have a lot of experience in this category. Recently we've secured a few by emailing the social chair, but other emails we've sent have simply not been returned, and we're not sure if they were received or not....Is there a more formal way (i.e a written invitation mailed to the house, etc....) that would have a better response? Is it better to contact friends in the sorority rather than the social chairs? The suggestions of both fraternity and sorority members would be very helpful here...

Thanks!