In a glass bowl, whisk together the egg yolk, salt, mustard, and sugar. Combine lemon juice and vinegar in a separate bowl, then thoroughly whisk half into the yolk mixture. Start whisking briskly, then start adding the oil a few drops at a time until the liquid seems to thicken and lighten a bit, which means you've got an emulsion on your hands. Once you reach that point, you can relax your arm a little and increase the oil flow to a constant, but thin stream. Once half of the oil is in, add the rest of the lemon juice mixture.