Making Weight in Taekwondo from www.ausport.gov.au

It is advantageous for taekwondo competitors to compete at the upper limit of weight categories. Typically, a 72 kg male will be more competitive if he can drop a couple of kilograms and compete in the 69 kg division, rather than fight at a weigh that is towards the bottom of the 80 kg division against much heavier opponents. However, athletes need to choose their weight categories based on competition goals, growth stage, body composition history and minimal weight for good health. In the long-term, the choice of weight category should be consistent with good health as well as good performance. In general, athletes should aim to keep body weight within 2-3 kg of the upper weight limit for their weight category. This means that the need to 'make weight' before competition will be kept within a practical and manageable target. Athletes should work with a dietitian to reduce body fat levels and maintain weight at a suitable level. Young athletes should also be prepared to move up into higher weight categories in accordance with their growth changes.
There are a number of tactics that can be undertaken in the last days before a competitive event to help to 'fine tune' body mass. In the 2-3 days prior to competition, athletes should avoid excessive salt intake, to prevent fluid retention. Adopting a low-residue diet for the last 24 hours before competing will help to reduce weight further, since it empties the gut of indigested food and fibre.

Low Residue Foods

low-fibre cereal (e.g. corn flakes, rice bubbles)
white bread
jam, honey
Clear juice, low-fat milk, cordial, sports drink
Pureed fruit
jelly
clear soup (e.g. chicken broth)
white pasta
white rice
tomato based pasta sauce without extra veggies
liquid meal (e.g. PowerBar Protein Plus powder)

Depending on their size and typical diet, most people carry about 0.5-1kg of such material in their gut during the day. Fasting will allow this food to be processed and eliminated, and cause a 'technical' weight loss. However, it will also prevent the athlete from fuelling up before their event. On the other hand, a low-residue diet composed of nutritious foods with minimal fibre or waste product will achieve nutritional goals while being 'light' to eat.
Traditionally, competitors use extensive dehydration to lower body weight prior to competition. Excessive dehydration can adversely affect performance and increase the risk of heat stress. It is smarter for competitors to manipulate food intake, then passively dehydrate over the day before competition. Passive dehydration involves limiting fluid intake while undertaking normal daily activities. Remember that approximately 500 g will be lost overnight as you sleep due to passive fluid losses. Use of saunas, sweat suits etc. should not be necessary if you have planned your preparation well.