Ann's advice is very good. It will also help him/her to remember your name! Important if you want him/her to write you a meaningful reference. I've had people like that write references for me and the problem is that these professors never actually get to know you! Therefore, their references can be a bit 'neutral' and 'bland'. When competing for jobs, you need an enthusiastic reference, not one that says, 'yeah, he's alright'