Not a capital defender, but worked in capital defense before law school and have a general lay of the land.
Basically, if you want to work for an organization that litigates novel death penalty issues in the Supreme Court-places like the Equal Justice Initiative or the Southern Center for Human Rights-you'll probably need a conventionally prestigious law school resume: top school (H/Y/S/NYU/Berkeley seem to be top feeders into this work), maybe a clerkship, and lots of demonstrated interest through internships, courses, and clinics (I know H/Y/N/B have death penalty clinics, not sure about S or the other T14 schools). There are lots of other jobs in capital defense though, and not all of them are as prestige-conscious: for all but a few employers, public interest recruiting generally is much more about demonstrated interest than it is about prestige. For those types of jobs, it's probably viable to go to the cheapest school you can in an area where the death penalty is used (see below), do everything you can that's death penalty related in terms of courses/internships/etc, and network like hell.
A few things to keep in mind:
- 1. You'll have to live where the death penalty is actively used. That means the states of the old Confederacy, plus Arizona, Ohio, and Missouri, for the most part. California and Pennsylvania may also be an option-they have a lot of death sentences but the clients never get executed, so the really meaty work tends to be elsewhere. Keep that in mind-can you/do you want to live in one of those places?
2. If you want to do anything on the indigent criminal defense side of things (capital or not), don't do any prosecution work! It'll be a huge black mark on your resume for a lot of criminal defense employers (interestingly, this is not true the other way around-prosecutors are fine if you've done defense-side work).
3. Make sure you actually like it. It's a really, really hard job. I went into my pre-law school job in capital defense thinking I absolutely wanted to be a capital defender and burned out very quickly. This is common. Think about a backup plan.
I wrote a post on this a few months ago that goes into a few things I didn't discuss here. Happy to answer any other questions you may have.