If you are like most people, you're finances aren't quite as simple as you'd like. Maybe there is that small balance on a credit card that you used to get your appliances interest free, or you keep a separate bank account on the side for emergencies, or that utility bill that only comes once every three months. Nearly everyone has something that causes them trouble occasionally. The important thing is to know where your potential trouble lies, and to keep on top of it.
I know that I have two trouble spots: a department store credit card that I almost never use, and the water bill on our house that is currently rented to tenants. There are a couple ways that I could deal with this: I could note the on my calendar to ensure that they get paid, I could be hyper-vigilant about picking up our mail every day (as we're overseas, stuff is already running late), or I can check online all the time. I've tried all those, and I've finally settled on a system that seems to be working so far. I call it my Once A Month Check-up. One day a month, (it's the 22nd for me), I quickly log on to all my bank accounts, credit cards, and that darn water bill. It only takes about 10 minutes if I don't get distracted, and I can ensure that everything is on order. Fortunately, there isn't too much trouble that can occur in just one month. I still open my mail, but this once a month plan makes sure that I have a tight grip on everything at least one moment a month.
If you have little details that seem to slip through the cracks of your financial plan, consider a Once A Month Check-up as a solution to your problem.