What are important courtesies to remember when asking for letters of recommendation?

I am applying to graduate programs and want to make sure that I am following the rules when it comes to asking for letters of recommendation. Some recommenders I have a very close relationship with (mentors, for example) so I am not worried about asking them, but there are others (lecturers, for example) that I am a little nervous to ask. It just feels so awkward because I haven’t spoken to them since I took their class (some up to two years), but I know they would give me a good recommendation. Also, I live about an hour and a half away from the University where most of these recommenders are. Would it be inappropriate to ask via email so long as I make it very formal? What are some other protocol I should follow here?