Check the work of Dillman (e.g., see link below), he has written extensively on increasing responses to surveys. Many of you have posted good suggestions in line with Dillman's recommendations like mailing reminders and so forth.
There is a chapter in his new book on internet surveys. It is not stellar, but it would be useful for someone new to the area.
http://www.psypress.com/shopping_cart/p ... 857528&pc=We've (my research group) have had success with mailouts (Canada Post mailouts by postal code), telephone interviews, and most recently we have internet data on 500 postmenopausal women.
Response rates have been falling over the years, however your response is quite low. However, this is difficult to judge since response rates will depend on who you are getting to respond (An undergrad class vs a random mailout, for example). So, if you are randomly sending out 230 surveys, 13 is low but not shocking.
Anyway, let me know if you want any specifics. Feel free to email me.