Old-fashioned paper envelopes because a stock image of a PDF would not be that exciting.
This post will not tell you how to build strong relationships with letter-writers, and it (obviously) draws heavily from my personal experiences. However, my questions may well be ones that you are wondering about as well.
Letters that I used:
1 PI letter: professor, mechanical engineering, did not take a class with
2 engineering faculty letters: mechanical engineering professor (course classified as Physics on AMCAS), bioengineering lecturer (course classified as Biology on AMCAS)
1 biology faculty letter: general biology (huge class) lab TA, cosigned by the professor, whom I have never spoken to in person
1 humanities professor letter: upper division class
1 clinic volunteering letter
With the exception of the biology letter, I regularly went to office hours and built up good relationships with these faculty members. They knew me well and were pretty eager to write a letter on my behalf (except for the mechanical engineering professor, who erroneously thought that only the only faculty that should write LORs for med school were PI's, and thought I was choosing him over his colleague whose lab I did work in...but explanation brought him around).
Why did I ask for that biology letter, then?
Engineering majors will probably know the answer, and I'll address it with the biggest LOR question I've had this cycle:
1. Most medical schools require letters from faculty in the SCIENCES. Do letters written by faculty in engineering with whom you've taken engineering classes count towards this requirement?
Put simply: does engineering count as science?
The short answer: YES. However, the internet is frustratingly vague on this front, and anecdotal evidence was not particularly helpful, especially since most of my premed friends were not engineering majors. While it seems absurd that engineering would not be considered 'science,' this presumption is furthered by the AMCAS course classification table that outlines engineering disciplines as falling outside of BCPM.
As an aside, I successfully assigned the vast majority of my bioengineering and mechanical engineering classes as BCPM (B and P), and a materials science course as C. However, I did assign my independent research class and a MATLAB class as 'Engineering.'
The caveat to this letters of recommendation thing is that this post is still anecdotal evidence at best, and I strongly encourage anyone who is concerned about their letters not fulfilling requirements to verify their validity on their own. After you are verified by AMCAS (hopefully you will submit early) or even earlier, call each school you are applying to and get the answer from the admissions office.
I'm not joking. As it turned out, almost every single school accepted my preferred four letters (PI, engineering faculty, and clinic), and I only used the weak biology letter and humanities one two or three times.
Working backwards in time, here is another question I grappled with:
2. When should my letters of recommendation be in?
A side questions: is asking for a letter from a spring semester junior year professor (if applying in the summer after junior year) cutting it close?
Letters of recommendation are needed to be 'complete' at a medical school. Though they are mentioned on AMCAS and will be sent to schools through AMCAS, they are not 'due' when you submit your primary. Since you don’t want your application to be unnecessarily delayed, you want your LORs uploaded onto AMCAS well before you plan on submitting any secondaries, just to be safe.
Because I was unclear on this (a very popular internet premed resource for the UC system spread information that LORs should be ready at the primary stage) I had my writers submit their letters to the Career Center letter service by the first week of June. Biology letter blew this 'deadline,' but I didn't use that letter much anyways. If you want to submit secondaries right as they come out (maybe you submitted AMCAS early, and prewrote like I did not), mid-June is a good ball-park deadline. Virtues of being early in this process aside, 4-6 weeks is a good amount of time to give someone to write a letter. Because UC Berkeley ends the semester in early May, I was able to get a letter from someone I took a class with in the spring semester after regularly attending his office hours over the prior few months.
And going farther back in time:
3. Who should I ask for letters of recommendation and when?
Who: a faculty member (professor or lecturer) who knows you well and can vouch for your intelligence and diligence in their class. Hopefully, you've gone to their office hours and asked insightful questions, and are able to have conversations with them about relevant topics not explicitly covered in the lecture material
When: regardless of if you take their class, ask for a letter after you've developed good rapport with them. Ask them around finals week or shortly before so you'll be fresh in their mind, even if it will be another year when you apply. The spring before your application summer, ask them formally for a letter of recommendation, and get on them to stick to deadlines.
Professors do this sort of thing all the time, and part of their job is to help students of the university achieve their post-graduate goals. However, this certainly does not mean you are entitled to a letter, and learn how to receive rejection or reluctance with grace.
As for how to get the letters to AMCAS, Interfolio is a good idea. I personally used the Career Center letter service, and avoided some (anecdotal) problems of delays and lost letters by doing everything electronically and early. I don't regret setting an early June deadline, and fortunately, I did not have any trouble with my letters this cycle. I've left quite a bit out, but these were just the points that I particularly wanted clarification on when I was at this point in the application cycle.
However, I strongly recommend an external service like Interfolio simply because I would like to have certain access to those documents in case I had to reapply.
4. Closing thoughts
I may have made a mistake in some of my faculty letters, but that comes from the fact that I was a wimp and seldom pushed myself out of my comfort zone to really get to know professors. I'm sure that one of my engineering letters, and most certainly the biolog letters were weak. I also wish that I had gotten a letter from my Spanish teacher as my humanities letter since foreign languages are high-effort classes that significantly show your participation and the way you handle unfamiliar material.
I wish I'd used Interfolio, but no harm, no foul.
Go to office hours.