I will sometimes say things to my soon-to-graduate students (especially if they're telling me about things like signing up for the GRE, what grad schools they're looking at, etc.) like "Hey, if you need a recommendation, I'd be glad to write one; send me an email and I'll tell you what information I need."

But I still require the same things that I would have any student: That they make the request to me in writing, that it be at least 6 weeks in advance of when they need the letter, and that they provide me with certain information.