I was originally going to write my long success story, but instead I'll keep this brief and point out some key things that I believe aspiring monkeys should know. I am in no ways an expert on breaking in, but believe I can at least provide a bit of direction. Quick background: Marine Corps background, Non-Target Accounting major, official bad ass.
Deadlines: For something so basic, this nearly destroyed my chances at investment banking. After an absurd amount of networking, I almost missed the deadlines with a fewand middle market investment banks. However, I was fortunate enough to secure a premier middle market investment bank due to one more opening. My point is to know when certain deadlines are due for resumes drops, interviews, etc. you can find them on some bank's website or around here.
Networking: This has been touched on 1,000 times and I still don't believe that it is enough. I was fortunate enough to have a large network of military veterans in investment banking. Networking can trump anything, low GPA, non-target school, rape charges, or anything else I can think of. If done properly, networking can be very effective. Look up how-to's here or on Mergers and Inquisitions about how to cold call, cold email, and ask to meet up for a drink or lunch. Create a spreadsheet on excel and write down names, banks, contact info, dates you talked, and notes about each individual. This is definitely a numbers game, so reach out to as many people as you can.
Skills: If/Once you're fortunate enough to get an interview; you have to know your shit. The technical guides from WSO, BIWS, and Vault guides are great for this purpose. When it comes to hearing what interviews are about, I strongly suggest reading Marcellus_Halberstrom's UG Recruiting Part I, II, and III. Know why you want investment banking, S&T, research, or whatever it is. Also, know why you want to do it at the specific bank. Once I had a convincing story, I was getting much more interest from interviewers.
Attitude: I'm probably the last person on earth that would believe in karma, but having a good outlook and being a great person can go a long way. I was at a Veterans On Wall Street affair and after the entire event of no luck I decided to leave. While leaving I held the door open for a woman who just happened to be a fuckin' BAUS at her investment bank. She got me in touch with people and later got me an interview at her bank...just for holding a door open. My point is be a good person with a great outlook and maybe people will want to surround themselves around you.
Work hard: Nothing comes easy. I reached out to easily 35+ people and I'm on the lower end of some other candidates. This is a pure numbers game. If you try hard enough, you'll eventually get in. Do not accept defeat and you'll go far in life.
I know everything here has been discussed over and over, but they work and they helped me. Network, familiarize yourself with the technical skills / knowledge, have a great attitude, and work hard. If you're a veteran I'll be more than happy to field any questions you have, but keep in mind I'm still a newb in this game. If you're not a veteran, quit being a pussy.
Good luck to all of you.
TL;DR - Network.