Ingredients: rum, lime, sugar syrup.
Shake, with ice, one jigger of rum, a third of a jigger of sugar syrup, and the juice of one lime. Pour, with ice, into cocktail glass. For extra fun, frost the glass with sugar by rubbing the rim with a lime wedge and dipping repeatedly into a bowl of sugar.
Blend the ingredients with enough ice to make eight ounces for a frozen daiquiri (you will need a bigger glass). Use four large strawberries instead of the lime and blend with ice to make a strawberry daiquiri. A half lemon instead of the lime is good too. I sometimes augment the sugar with a bit of triple sec, and if you like to do the tropical thing, Malibu (coconut-flavoured) rum adds some spin.
My preference is to use real lime juice, but you can make a great daiquiri (and some say a more "real" daiquiri--see also the margarita page) with one jigger of Rose's lime juice and a splash of triple sec instead of the lime plus sugar syrup. Only use Rose's though; her low-rent competitors make a bitter product.