Mine was eating out and frivolous spending, like Target. I could always find a few little things that I need, but it totally adds up. We ended this by using only cash for non-essential things. We take the cash out for eating out (it's our "entertainment fund" and has to cover all fun extras for the months) and shopping (we each get an allowance that we can do whatever we want with) and when the money's gone, it's gone.
We don't use the check card or credit card for anything non-essential. I could kill our monthly budget with little $5 - $10 nothing purchases using the check card because I didn't even really count it in my head, but if you do that every day, it's a problem! The cash only system works SO well for us.
Technically, I could go crazy at the grocery store each week too, but we have a budgeted amount and force ourselves to stay within that amount. We live on one teacher's salary and we all know hard that can be to do in Southern California!