1)When you are caramelizing the sugar don't stir it, else it will crystalize the sugar. Just swirl the pan.
2)When you are simmering the sugar for the final stage, cook it on a very very low heat.
3)Adding lemon is important, the citric acid in the lemon will prevent the sugar from getting crystalized.
4)If have over cooked the syrup and once it cools down, if it gets too thick than needed, don't worry, empty the sugar in a pan and add more boiling water and simmer for couple of mins.
5)Store this in a clean, dry heat safe jar in normal cupboard and not in fridge.