Since my weight loss journey, I've been keeping a spreadsheet and updating daily my weight, calories eaten, etc. And it's been a really useful tool, especially the graphs. I can definitely see a relationship between calories eaten and weight loss.

In addition, I also make note of everything I eat throughout the day. All this data collecting takes a some time and effort( not too much ), but I believe it's worth it because it's so interesting to look back and see what happens when you eat this, and that. The only two things dieters have in control over our weight loss is how much we eat, and how much we exercise. So I do find it interesting to tinker with these values, and see the net result.