I have taken a staggered approach to direct mailing. I e-mailed out about 90 applications/resumes/cover letters to various firms, and also bid on about 30 firms through my school's OCI. In the next two weeks, I have about 10 interviews through OCI and 2 callbacks, both of which came from direct mailing. The odds aren't good (2 callbacks vs. about 30 rejections so far), but remember: It only takes 1 offer to get you a job.
All of the substantive things (cover letter, resume, etc.) should be in the form of .pdf attachments to your email. The body of the email itself need say no more than your name, that you're a 2L at [Law School], and that you're interested in a 2011 summer associate position. One obvious thing: Name your files something simple and professional, such as Name.Coverletter.pdf. DO NOT name your cover letter files according to the city you're sending it (i.e. Coverletter.Boston.pdf, making it obvious that you have one that goes to all firms in a particular city and that you're applying to other markets). Bitcheslovemyshaft.pdf or similarly absurd/random filenames, of course, are a great idea.
I think that if you haven't already sent out mailings, having success is going to be an uphill battle. That said, if OCI doesn't pan out, I will be sending out another batch of 100 or so direct mailings in another month, so I'll keep you posted.