How to stay consistent: a plan for your busiest weeks.

How do you stay consistent when the enthusiasm lasts three weeks? In January you're back at it: gym, lighter dinners, early nights. Then comes board-meeting week with two flights, and by February you're back to square one.

'I'm just not a consistent person' is something I hear a lot. Willpower usually has little to do with it. The plan was built for quiet weeks, and if you run a business, those are rare.

How to spot a plan that needs work

• You start with everything at once. Five sessions, a perfect diet and early alarms.
• Your only goal is an outcome. A stone lighter by summer, and the scales judge you.
• There's only the full version. And in a packed week, nothing wins.
• You switch methods every month. The latest diet always looks like the one.

What happens in week three

Two different engines. The early push comes from wanting to change. What keeps you going comes from results and habit. In between there's a gap, often between weeks three and six. It's hard and it feels like you're getting it all wrong. Usually that's the old pattern breaking down.

Willpower depends on the week. On holiday the plan is easy. In year-end week it takes far more. Build it for the hard ones.

Under pressure, you go back to who you think you are. If deep down you still think 'I can't stick at things', a heavy week brings the old habits straight back. Habits last when your picture of yourself changes.

How do you stay consistent with exercise and diet?

With a few small habits at a fixed time, and a short version decided in advance. Turning up often beats doing a lot.

Start with less than you can manage. Two sessions a week: people who come back with six usually stop early. One new thing at a time, the second once the first feels automatic. And where you can, do them early. Evenings usually find a reason to skip.

Goals that are down to you. Next to the number on the scales, write down actions: 'savoury breakfast every morning'. You can do an action even in your worst week. The result comes in its own time.

The short version, decided with a cool head. For each habit, decide what you'll do when the day blows up: ten minutes instead of the full session, a main with vegetables at the restaurant. Something beats nothing.

Never miss twice. It's a rule of thumb. A skipped session or an unplanned dinner out happens. Two in a row can be the old habit creeping back. Turn up for the next one without punishing yourself with double sessions.

Plan the impossible week. Trips, quarter-end, the festive season: you know they're coming. On Sunday, check your diary and mark the lighter days. A lighter week keeps you in the habit. A three-week break sets you back.

Choose who's around you, and keep count. Ask who gets in your way and find an ally, a friend or a group who pulls you along. Note every time you turn up and compare yourself with you a month ago, never with anyone else.

Tonight, pick one habit and write its short version. When a week goes sideways, it keeps you on track.