What is the best way to get 6-pack for an ordinary person in 1 month?

If you have a gut, or slight pudge in your tummy, and can't get your hard work at the gym to shine through, diet is the cause.  Eating a high lean-protein, high vegetable low carbohydrate, low dairy an no sugar meal plan (like paleo) will help you see the results of the gym quicker, reduce your body-fat percentage, and lean out.  Strength training is the best way to improve a 6-pack. Although cardio will help shed the fat, muscle is what makes a 6-pack.  Exercises like sit ups, planks, toe-to-bars, knee-to-elbows will help build your core. Think about joining a gym with either personal training or group training. Reputable CrossFit gyms will have coaches there I help you meet your goals. Results don't happen immediately, but with a strict diet, and exercise, you will see results quicker than not.

Everyone has an awesome 6 pack under the 'hard to shift' layer of fat most of us carry around.  Being nearly 50 now and only having seen my abs for a couple of years between 30 and 35 and can I describe a regime of high protein, low carbs and medium level of 'good fats' (linseed oil, fish oils etc) as the way I achieved this goal.  Also I have been weight training for over 30 years so believe in the simple adage of less in and more out in combination with correct and sensible eating as the only way to achieve this look.  Good luck and it is a journey worth taking.

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P90X3  Get it a full work out in 30 mins.  Lot of excellent core work and cardio in every DVD.  Only need a few weights; PowerBlocks works great and a band for pull ups and you are ready to go.  Also, cut out the processed foods and sugar and get good sleep.

Work out your basal metabolic rate of calorie intake. Then half it. Stick to that number, but make sure you take vitamin D, B and E  Eat clean carbs 1 hour before you train and first thing on a morning. Increase your calorie intake to 500 above the prevoious 7 to shock they system. In 3 week you can easy loose 4-5% body fat if you train hard. No sugar, sweetener, dairy and good fats (fish, avacado).  Strength training disent make abdominal alone. You need to vary your training. So a 20 minute abb circuit after every season will force the Abbs to grow. Don't forget they are used all day everyday and throughout every training session. By overloading you will force growth.

Just visiting the gym for a whole day..I am yet to see somebody with such amount of energy. In other words, Rome was not built in one day and so you should just be patient and maybe do two or three hours of gym every day for the next six months or so...