We didn't do the dining plan in 2008 but in 2007 you were given two receipts. One had the total on it and you signed it. The other had a zero on it and you got to keep it. I just wrote down the total amount on my receipt and when I got home I totaled them all up. We definitely saved money with the dining plan.
We used the DDP for the first time last week and wondered if it was good for us or not. I kept a pen in my fanny pack and wrote the amount on the top corner of the reciept. At the table service you get two reciepts, one with the amount and one not...but for the snacks and counter service you don't. So find a small sharpie or something and tally them up when you get home!
we did and found that we saved money!