We were buying groceries or meals 40(!) times per month, so we set our goal to cut that number in half. Using these rather large number stamps left over from the table assignments at our wedding, blank Rolodex cards and a freebee fridge clip from a community event sponsored by Washington Mutual, we numbered the cards from one to 20, in descending order with 20 on the top.
Whenever one or both of us goes grocery shopping or buys lunch, say, we take the top number and stick it on the bottom of the pile clipped to the fridge. When the last numbered card has been rotated to the bottom, one extra card remains, and we know we have to wait until the following month to buy any more food.
That last card? It reads "rabbit, rabbit." This reminds us of our mantra, with which we begin each month: "Rabbit, rabbit, success and progress for this new month of ---." We've found the process motivating and effective.