Keeping a food diary is my #1 tip.
It takes a bit of planning, but I keep mine on a Word doc on my work PC, and on weekends i just jot it down on my iPhone then email it to myself. Easy.
I started out because I was doing all this execrise and not losing any weight and i was like stuff this, i'm going to go see a doctor. I figured I would write down everything I eat for a month, then take my little list to the doctor and he can figure out how come I dont lose any weight. I thought there would be some myseterious reason!
Turns out, I ate way more crap than I actually thought I did! A 'handful of potato chips' was actually half a bag, I'd eat a pavlova for dessert and convince myself it's ok because it was garnished with half a strawberry and a slice of kiwi fruit!
In that first month, I was thinking to myself oh crap if I eat this I'm gonna have to write it down!
Also reminding yourself how many minutes you'll have to do on the treadmill to burn off those treats.