I don't ask. I just preemptively give out boxes of homemade Thank You cookies. I put it with a note that says, "Thanks for helping me out with my application!" Then, if they haven't done it yet, it's like extra incentive. Or a guilt trip. Whatever the case, it works! ANd if you can't make cookies, give out 10 dollar Barnes and Noble certificates or something.
Yeah, I just either send them a thank you or send a polite email asking them if they need any more information. Either way it's not as if you're demanding the letter.
Except when they eat all your cookies and still don't complete your LOR! Okay, okay, I should chill out since I told them to have it done by Oct 15. But I thought that they might want to do it early since the cookies were so good.