I keep a daily food journal.
I write down everything I eat and the calorie content. If I am eating at a friend's house or do not have access to the exact number of calories, I make an educated guess.
If I eat out, I try to look up the amount online. (You might be surprised how many restaurants list their nutritional info online).
I do not go overboard or get obsessed with keeping track, but it is a great way to see what foods I'm putting into my body & what foods I eat too much of.
Plus, I also log how many miles I ran that day and subtract how many estimated calories burned from the total number of calories for the day. That gives me a good estimate of how many calories I ingested for the day, after exercise.
You can do a food journal short-term or long-term. Some folks only need to keep track of what they eat for a short while, and they learn all they need to learn about their eating habits. I have heard, however, that those who keep food journals longer lose more weight. It makes sense to me, because they are keeping themselves informed about what they eat.
My food journal is also small enough to fit into my purse, so it's easily transportable!