I went from a size 16 back to a 12 in a year, id put on weight while on depo prevera and thought id never lose it, and tried the usual diet thing but it didnt work, then I done some research and got some really handy hints that worked for me!
Apparantly thirst can be mistaken for hunger which is when we are most likely to snack, so always drink a pint of water first and see if that helps.
Someone who sits down all day (ie office worker) is more likely to put on weight, not just for the obvious reasons but becasue your system thinks you are lighter than you really are so increases your appetite and slows your metabolism. Standing up for atleast 20 mins in each hour and walking about corrects this so your appetite and metabolism are correct for your weight.
It takes your stomach about 20 mins to realise it has been filled so if you eat quickly you are more prone to over eating and feeling too full - eat slower and take smaller bites (let alone smaller portions lol)
And also small changes can make a big difference, swap all your diary products like marg, milk, cheese, mayo etc to low fat (vegetarian cheese rocks!)
Other than that I just used the old but good eat sensibly and stay active.
Good luck!