Ok I think I really nailed this mass mail thing down so I thought I'd contribute to the (wealth) of knowledge in this thread. I did a small campaign, hitting about 50 random nyc firms, and got five cbs out of it. I have good stats but not like incredible stats so I think my mailing tactics might have had something to do with it. I also have a bit of experience in email marketing so I'll share a few things that are kind of basic there but I havent really seen itt.
First thing I would advise: download an email tracking app. These will put a pixel into your emails that notifies you when the email is opened (and the good ones will tell you when it's forwarded or when an attachment is open, but few of these are free). Yesware is a good, easy, free program.
This is crucial for any campaign. If nobody opened your email after a week (which happens at a majority of firms that put out catch-all addresses, in my experience) that gives you license to hit up an individual. If someone opened your email but didn't open it again, forward it, or contact you for a period of time, that gives you license to try again with someone else at the firm. I hit up the nalp/firm website contact first. Failing that, one way or another, I hit up every recruiter I could find that was employed there. (I think it is important and a sign of respect to the process to start with this, but whatever, you need a job). Following that, I did practice group leaders and hiring partners (one by one). I got two cbs by getting to the third rung. I don't think hitting up associates who went to your undergrad/law school is fruitful by comparison, but by all means try if everything else fails and you really care about the firm. If you are not tracking, it is guesswork as to if you were noticed, and if you were noticed and still applied up the chain, it's not a good look.
Programs like yesware will also allow you to send the email at specific times of day. Do not underestimate the importance of timing in an email campaign. Aim for half an hour after an attorney or recruiter, respectively, generally gets into work or has completed lunch, and are thus groggy and has just caught up on email. If you're getting in when they're not at inbox zero, you're more likely to get glossed over. Get them when they're in zombie mode and just got rid of their mess of unreads, in other words.
Maybe not relevant for this cycle, but circumventing the oci process and reaching out to recruiters by saying you will be in town early and want to come in for an interview then instead of waiting for oci (because it is a top choice firm!) also got me a direct to cb cb. Works out for both parties; they don't have to pay for you to take a cb, and they get an extra screener slot at oci. Make sure to use a school specific and thorough subject line to make it seem like you'll be asking something logistical about oci.
Another thing for folks who might be a tad more on the risk-taking side of things: it is a classic spam email tactic to put "Re:" at the beginning of your subject line. It catches your eye because it looks like it's a response to an email you sent out. Luckily for you, you also picked a profession that uses "re:" in a way that doesn't sound like spam when your email subject line is "Re: Summer associate position". I didn't do this, but I was really tempted to, as it definitely works.
Some firms don't post email addresses for recruiters. It might not occur to people on how to get someone's email address, so just look at a partner's email address at that firm. Pretty much always going to follow the same pattern. But then a fair amount of firms don't post email addresses for anyone and require you to submit an inquiry through the website. Google "site:[firm website.com] mailto "@[firm website.com]" [specific contact's name]". You'll at least get the root pattern to work from. Short of that, eliminate the "site:" seach operator; it'll be out there somewhere.
Also keep your cover letters really really short. Nobody has time for that. You should be able read pretty much the whole thing without scrolling on your phone. Down to very basics. Don't bullshit anything or explain anything; be curt. You have their attention for five seconds tops, make it count and cut out the formalities. Among these formalities is stating the full firm name. They already know it. If you're mming skadden say skadden; don't waste that real estate with arps et al. Also among these formalities is the block of contact shit below your name. Don't use extra text anywhere. You already put your preferred contact info in the last sentence of your message and put your school in the first; don't repeat it. Your name will do. (And that sign off should also be the first place your name appears).
Also make sure your resume is the first attachment. That's what they care about; make sure it doesn't take an extra half second for them to get it. That can be the entire difference.
Also absolutely don't do anything that makes your email look like it was a mail merge or was an automated send off. Do the mail merge in word and copy and paste it over using notepad to clear the formatting (unless you have a lot of cash to burn on a pro program). Yeah its a bitch, but include the email address and subject line in your mail merge so you can get a really repetitive process that only requires two windows and alt-tab mastery. Won't take too long; we're not talking about sending 500 emails here. Should also go without saying but follow the one person, one email rule. Sending an email to two at one firm is sending it to zero, and bccing to multiple contacts is just as bad.
So that was kind of scattered and there are probably more specific things I can't think of at the moment, and my flight is about to land, but hopefully that helps someone.