My biggest piece of advice? Don't call it or look at it as a diet! Look at it as you changing your lifestyle, forever. My biggest bits of advice would be...
*Join a website like myfitnesspal.com where you can track your calories and exercise, guessing doesn't work, you'd be surprised how easily you'll get it wrong! They've got apps for iphones, androids and blackberrys too.
*Make yourself accountable to someone. The best thing about groups like Weight Watchers is that you know that you've got someone weighing you, you can't just pretend that you've done well that week when you've eaten everything in sight. So maybe try and buddy up with someone, or even just start a diary on here. This site is full of amazing supportive people!
*Take a look at those foods that are your favourite, see if you can find a more healthy alternative so you can still have the things you like.
*Don't deprive yourself! The best thing about calorie counting is that anything is allowed, it's just about moderation and working it in.
*Make sure you allow yourself enough calories, you don't want to do yourself any harm. Check out this thread to work it out:
http://www.minimins.com/calorie-counting/202903-answer-how-many-calories-should-i-eat.html
*Set yourself goals and reward yourself when you get there. If, like me, you've got a lot to lose then having just your end goal isn't enough to motivate you because it seems so far away. So break it down into stones or something like that, just take it a step at a time. And each time you get to goal, reward yourself even with something little like taking the time to give yourself a manicure or something. I know it helps me.
And my final piece of advice? Don't beat yourself up when you have a day that doesn't go according to plan. We ALL have them, I've had a lot of them lately! Just remember that this is a journey and it's got lots of obstacles and sometimes, they'll get you but all that matters is that you get back up and carry on. Guilt and worry serve no purpose and you haven't failed or lost until you give up, so just get straight back up and carry on.
Good luck