If only losing weight were like losing your car keys: frequent, effortless, and guaranteed to happen just when you have someplace important to be. Instead, excess kilos have a way of spontaneously combusting when dieting is the furthest thing from your mind – when you're in a new relationship, knocked flat by the flu, swamped at work. Although muscle is usually the first to go in those cases, an unexpected loss of two to five kilograms can be a good starting point for reaching a healthy weight.

Of course, if the cause of the drop was unhealthy or impossible to maintain, it’s going to take more than wishful thinking to keep the weight off. Give these expert-approved strategies a try...

There’s A New Man In Your Life

A big dinner doesn't seem so appealing when you have butterflies in your stomach. "When you're nervous or excited – as you are at the beginning of a relationship – the brain releases certain hormones, including adrenaline, that can suppress appetite. That's terrific – until a few months later, when your nerves calm down and your appetite ramps up. And the more time you spend with your honey, the more likely you are to pig out: according to a study presented at the 2007 annual meeting of the Obesity Society, women in cohabitating couples have higher obesity rates than singles, and newly married women gain an average of 10kg over five years (the average weight gain for single women is 7kg).

Keep It Off. Your guy might be able to bounce a coin off his abs, but don't assume you can – a guy burns about 2500 more kilojoules per day than his better half. If dates typically involve a knife and fork, plan them around physical activities instead – swimming, hiking, cycling or hitting the gym. Having a workout buddy boosts your chances of dropping a size or two.

After you've been sick as a dog for a week or more, colleagues' comments about your sudden slim-down can leave you lovin' your new bod. But don't buy a new wardrobe just yet: "It's unrealistic to expect to maintain 100 percent of illness-related weight loss, because not eating is not an option. Chances are, the only exercise you did while you were sick was crawling back and forth between the bathroom and bed, so your muscles have begun to shrink. A study in the Journal of the American Medical Association found that just two weeks of decreased activity can lead to more belly fat and less muscle.

Keep It Off Get to a gym. For every kilogram of muscle you pack, your body burns more kilojoules daily – and that's even when you're not exercising. And remember this: one kilo of muscle takes up a lot less space than one kilo of fat. So even though you might not see the difference on the scale, you will see it in the mirror. Just wait until you feel at least 80 percent healthy and have a green light from your doctor before pumping iron again.

Get back into it with three 30-minute full-body-toning sessions a week. To build lean muscle and tone without adding bulk, do lots of reps (three sets of 12 to 15 per exercise) with the heaviest weights you can lift while still maintaining proper form. Try doing two moves for each major muscle group (for example, leg curls and dead lifts for your hamstrings, push-ups and flies for your chest).

You’ve Been Stressed

When your calendar is at maximum capacity, you can be too distracted to eat. But when time, yoga, or your massage therapist finally kicks stress to the curb, your appetite will probably return with a vengeance. When it does, it'll be tough to keep the pre-burnout kilograms from piling back on because you've picked up eating habits like skipping meals and replacing your lunch with a Bar One and more coffee.

Keep It Off Since you've been too busy to shop, your cupboards are probably bare by now. Use this opportunity to rebuild your diet from scratch. Pack your kitchen with the nutrient-dense foods your body needs – whole grains, fruits and veggies, low-fat dairy, lean meats, and beans – and leave out everything it doesn’t.