Have you been lying in bed, sitting on the toilet, driving, brushing your teeth, in the shower or standing on the scales, and thought, RIGHT. This is it. Then you mentally make a list of all these things you're going to do and how life is going to be oh so much better and you're excited to really start living. Then a few days, a week, a month later you can hardly remember that random burst of inspiration and you are back into your old habits.
These random bursts of inspiration are usually triggered by something; you've read a good biography, seen a motivational speaker, been to a funeral, had a near death experience...
What you need to do, before making any changes to your lifestyle, is work out what motivates you and how you can remain motivated for long periods of time. Definitely easier said than done, but I'm going to list a few ways that can help.
1. Inspirational quotes. Be one of those daggy people that print them out and stick them somewhere in the house. Change it semi-regularly so you don't get tired of it. I found one I like:
"First say to yourself what you would be, then do what you have to do." - Epictetus.
2. Look at your family medical history. This is interesting and in my opinion not enough of us actually spare this enough thought. If there are hereditary conditions that you can lower your chances of getting, that can be a big motivational factor. It's too late when you're 60 and think, I should have done more when I was younger. Especially with weight-related conditions such as type 2 diabetes, hypercholesterolaemia, hypertension, arthritis (these are not necessarily caused by obesity though, I might add).
3. Get a health buddy! If you have a friend who is also keen on living healthier, have regular catch-ups and see where you each have gotten since you last talked.
4. Set goals. We hear this all the time, but that feeling of smashing your PB is a real rush. It helps to have lots of little goals, with a few big ones on the horizon. I remember when I used to run a particular route around home that had lots of hills around it, and I would always walk up the hills. The first time I made it home without stopping to walk I was so happy, and so motivated to take it further.
5. At the end of each day, ask yourself, has this been a good day? Not in terms of what you can't control, like the weather or someone getting you down, but more 'are you happy with the choices you made today'?
6. Give yourself presents as a reward, but don't use what you've been trying to avoid. Eg, don't think 'I've been eating well all week so today I'm going to treat myself and have McDonalds'. Keep your rewards healthy like a massage or a little shopping spree.
Obviously, everyone is different, but believe in yourself, believe in what you want to achieve and you will achieve it. Positive thinking is a powerful tool and you should use it each and every day.