I actually emailed them.. Told them my name, what course I took from them and what I received and how frequently I went to office hours/things to remind them of me! And I asked them if they were willing to write me a STRONG letter of recommendation. Also, I asked if I could formally meet with them to discuss this. And when we met and they said yes, I handed them a folder with my resume/CV, transcript, personal statement and waiver forms. Lastly, I told them that they could either send it electronically when they receive the email or send it back to my school's LOR service with the pharmcas evaluation form, which in that case would be paper....
I actually didn't talk to one of my organic professors all that much since he taught like 700 students... (2 lectures...) and had the same office hours for both classes.. -.- But he was very nice in that he set up a meeting with each student to get to know them while writing the letter of recommendation so it was all personalized!
Only downside was...you had to have gotten a A.... that certainly narrows it down... Haha
Hope it works out for you!