I actually got very lucky with my LOR's. I had 6 of them written and asked my writers last January giving them a deadline for last April (but really I told them they were due a few weeks before I really needed them). I gave them 4 months notice and each month I would write them an "update" email reminding them when I needed to turn them in to my schools premed committee. I had one procrastinator, but when the date came around I told her she had another 2 weeks so it was no big deal. Its not hard to write a nice email that has a little reminder in it. Just ask how their semester has been, remind them of the date and then write a few sentences about how you appreciate them taking the time to recommend you and how you know they are busy and appreciate their kindness, blahblah. You get the point. Good Luck...