It really strikes chord with me when I speak to people who say "I wish i could do that" or "I wish i had your will power". I don't want to sound patronising, but it makes me so sad as I used to feel exactly the same when I heard about people losing weight. Now I know that I can do it, that I could have done it sooner, and that I continue to do it every day, which shows that those who doubt themselves really can do it too - in these situations I just want to find a way to make people believe in themselves.
The classic and rookie mistake is to have one "naughty thing/meal/moment" and think "sod it, today/this week is ruined now, I'll have what I fancy now and start again tomorrow" and then proceed to eat everything we have craved for the past week or so, or, eat everything that you we going to deny ourselves over the next period of time when we are 'being good'.
When speaking to my friends about things like this I always find relating sensitive or emotional situations to basic every day examples as the easiest and most tactful way to make a point. In the instance of the rookie mistake mentioned above I suggest people liken it to a vase of flowers, if one individual flower died would you throw away the whole bunch? Of course not, you'd pick out the dead one and then continue to appreciate the others. In a similar way, just because you have one slice of cake or one takeaway, it doesn't mean you need to throw away all your other opportunities to be "good".
This goes hand in hand with the justification thought process we seem to build up as we tell ourselves that one bad day can be cancelled out by not eating enough the next day. Let me tell you a secret - we are going to have days when we eat more than others; we are going to have days when life gets in the way of our good intentions, and we are going to have days when that call from chocolate/kebabs/wine are all too tough to ignore. The trick is to accept these and plan for them. You don't have to feel bad for living, and one of my biggest things when I was first losing weight was that I did not want to have to put my life on hold until I was lighter. I wanted to be able to enjoy life and I wanted to be able to have a nice meal out without feeling guilty.
So remember, next time you think "ooooh I'd give anything to be a few lbs lighter" just pause and reconsider that statement. Would you really? Absolutely anything? If so, put the chocolate down. If not, then that's ok, just don't beat yourself up about it...
(just to add, this is of course all my own musings and is purely relating to the experience I had and that which many others have shared with me. I know there are people who have problems losing weight for reasons such as medical conditions, and in no way am I ignoring or belittling those challenges)