I have to faithfully use my journal. Otherwise, I just drift and use "selective memory"!
Some days I don't get all my milks in, other days I don't do enough vegetables. (I'm on WW.) With my journal, I'm able to look back over the past few days and easily see where I came up short and try to make up for it. It's also a really good way to find days where I did a good job sticking to the program, and then I can duplicate the menus from that day.
I carry my little WW kit with me all the time, with my journal, the points counter, and the food guides. I take it to work, to the grocery store, and anytime we eat out. I try to figure the points and write everything down before I eat it.
Journaling is really the only way I can honestly and objectively see how well I'm adhering to the program.