First I think I benefited coming from LAC, as there is A LOT of opportunities to interact with profs becuz they have nobody else to be busy with!
I have been with one prof (as RA & TA) since my freshman year, plus a couple of independent studies/research papers/honors thesis. So, that letter from this prof is spectacular!
Next, I do well in classes, and talk to prof very often, in terms of my goal to go to grad sch, and also potential careers. Profs in my school love to talk to students! (even more so especially when I say I wanna go grad sch)
However, one downside is that probably profs from LAC are less well known, thus harder to push you into top 10. Also, I think a slightly more unknown LAC will suffer more becuz of branding effect.
If you are in a top 20 PhD granting universities, I think the best way to get stellar letters is to do well in his/her class, and go to office hours to ask not only questions about courses, but also guidance for grad sch. I think they will be willing to help!