There is no big secret to losing weight or getting into shape – you just need to eat less and exercise more. But it is hopeless setting out on any weight-loss or fitness regime if you don't first set yourself a series of goals. However great your intentions, or the diet and exercise tips that you are going to follow, it is all too easy to lose heart along the way. But if you measure your progress against clear, attainable goals, each defined by a period of time, you will find that your determination to succeed increases tenfold.

The first thing you need to decide is why you want to lose weight or get fit. Perhaps you have signed up to run a marathon, or want to slim down a clothes size before the summer, or are keen to lose weight for your wedding day. Work out which day and which year you want to achieve this goal by.

Divide your goal into three categories and write them down – short-term (the changes you are going to make to your diet and exercise regime tomorrow, and in four days' time, and next week); medium-term (the way you want to look or feel by a certain time, and how you intend to measure your progress), and long-term (your ultimate goal and date).

Type these goals up, print them out, laminate the print-out and put it up on the wall, where you can look at it every day. Without doing this first, there is no point in starting to think about eating healthily or exercising. You just won't get to where you want to be.

Jon Roberts is a personal trainer with Matt Roberts personal training, London.