There’s a reason the gyms are packed in January and empty by February, and it’s got nothing to do with the weather. What happens during this time to make us go from keen-to-get-in-shape to lazy and gym-shy when bikini season is only four months and counting? It’s the same thing that makes us ditch our weekly fitness plans in any season—not so much motivation, but a lack of proper planning. Read on for some strategies to help you stay swimsuit-fit year-round.

Know what you want. First, decide what you need fitness to do for you. For instance, you want to be thin (but not Kate Moss-esque) or you want to be stronger (think: arms like Madonna) or maybe you want to build up your endurance so you can run your first marathon with your boyfriend. Now, set your starting point. Get a fitness test at your gym, time yourself running your fastest mile, or weigh yourself and measure your waist, hips and thighs at home. You get the idea. Hide these numbers in your bedside table for one month (don’t worry—only you will see them). You now have an idea of where you are.

Keep a food and fitness diary. Write down everything you eat for a few days and one weekend day, including the milk in your coffee and the food you pinch off other people's plates. Start looking for opportunities to make healthier, lower-calorie substitutions in your diet and to be more active whenever you can. Now act on them.

Fit it in anywhere. If you don't have time for a conventional workout, try lifestyle exercise such as walking, gardening and climbing stairs. It sounds daft, but keep in mind that during a normal day you probably walk between 4,000 and 5,000 steps. With a little effort, if you take a longer route between work, school or walking home, you can probably get that number up to somewhere between 6,000 and 7,000. There’s no need to count steps, just walk whenever you can.

Break up your goals into stepping-stones. Set small monthly goals. For instance, do whatever kind of exercise you like for five or six days a week for one month, then try out a different sport the next month. The more realistic your goal, the more likely you are to achieve it.

Write it down. Just as you plan to meet friends or go on holiday, commit to exercise and make fitness a daily priority. If you keep a diary, write down when you will work out, and remember, this doesn’t have to mean spending an hour at the gym. Don't rely on exercising on-the-fly or “whenever you have a minute,” as these windows of free time seem to disappear quickly.

Have a look at the numbers. After one month, get out that piece of paper with your measurements or running time. Now, measure or test yourself to see what’s changed. This is a new starting point for the next month and also tangible proof that you're either doing things right or that you need to work a little harder. Take this opportunity to think about your workouts and what you can do more effectively or what might be more fun. Make your changes for the upcoming month.

Treat yourself to something special. This is the fun part. If you've stuck to your programme, reward yourself with that pair of trousers you've had your eye on or enjoy a night out at the cinema. Do whatever makes you happy as long as it doesn't undermine your success—a double hot fudge sundae is probably not the best reward for sticking to your workouts, but you can have a biscuit or two. You deserve it.

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