^^^ Definitely teaming up with a sorority is the way to go. Your frat bros might think it sounds lame, but chicks really dig sappy sweet things. My frat every month or so picks a sorority and goes to their house and sings good-old-college-romance type songs, and they really love that. Or, if you have anyone in your house who can cook decently, you can really impress the ladies by inviting them all over for dinner. Group community service is good as well; often a sorority might have different connections in the community that you, and guys are always more motivated when there are chicks to impress lol.
Also, it's not necessarily bad to not be the greatest party house on campus. My house certainly isn't the most happening, but girls who come over repeatedly tell us that they feel safer here than at other houses.