It is easy for us to forget about the small things we do on a daily basis. Not only that, but there are two ways that this can effect us. For one, forgetting about the handfull of M&Ms you ate at the receptionist desk, and the half a cookie you ate with your afternoon coffee, can negatively effect your training/weight loss goals. The second is forgetting that you take the stairs 8 flights each morning, afternoon and evening, or that you walk to work rather than drive.
My point is this, you might not drop those last 5lbs if you keep forgetting about the M&Ms and the cookie, and you might not realize the actual difference of taking the stairs or walking on a daily basis in addition to your regular workout routine but both add up.
The best thing to do is to keep a journal of your nutrition and daily activities. If you write down everything and don't end up with selective memory, you might notice a pattern, or realize you snack poorly far more often than you ever remember.
This isn't a task you need to complete for the rest of your life, unless you really want to, but it is a great task to do for about 6-8 weeks every 3-4 months. Get yourself in check, and if you notice you might be slacking again in a few months, simply start keeping the journal again. There's nothing like stepping back and taking a look at the whole picture.