Well, folks, I never said retain one in advance, just to have a relationship with one who you KNOW to have extensive trial experience, because with your future depending on it that's what it will take.
Ideally, you'd have a small-office lawyer near where you live. Ask around the LEO's in the community to get a lead or two. Perhaps make an appointment (and pay for it, if they charge for a first consultation) and get to know them a little bit. It's good to have someone who has something of a general practice so that they can do some more routine work for you, since that helps to keep the relationship going. But it's essential that they have extensive experience in actually trying matters, not just settling civil cases or pleading criminal cases, because you just can't develop that skill set and state of mind over night. I think a minimum of around 250 trials or more would be a good idea.
Civil trial experience counts for a lot here, because if you have to defend yourself you may well be more at risk from civil liability than criminal charges, or you might face both.
None of my approach is original. I read an article a couple of years ago where Masad Ayoob suggested the same thing, and I've heard a couple of firearms instructors around here advocating the same thing. I put my plan in place by consulting the lady who does estate stuff for my family, and who I've known for years. She's a former 10-year prosecutor (major felonies) who still defends criminal cases as well as doing civil trial work. She even likes to shoot as well.
Now, we're each other's lawyers.
I'm sure some of the pre-paid legal work plans do okay, but I've never seen one handle a shooting defense. Some of the prepaid group work I've seen done in other areas of the law has been discouraging, to say the least. YMMV.