Make an appointment to "discuss future career goals." Don't just drop in, as you want some time to chat the faculty person up and remind them of your class contributions, any other interactions that might make for good LOR fodder, and personal factors that might be good to include in a letter (eg, like how you still made it to lab with a broken leg). Don't wear your jeans with the holes in them. Bring with you a copy of your unofficial transcript, a CV, a non-quirky photo, a draft of your Primary Statement if it exists, and printed instructions about where to send the letter. It is beneficial to look organized. Ask if they feel they could generate a strong letter in support of your candidacy. If they say yes, leave the materials above so they recall who you when they get around to writing the LOR. If not, thank them for their time and consider someone else.