Thinly slice the steak. This tends to work better if the steak is near frozen.
Mix the beef with the corn starch, half of the soy sauce, half of the sherry, half of the sugar, and the sesame oil. Mix this very well, and allow to marinate in the fridge for about half an hour.
Heat the oil in the wok till about 300 (yes this is low, so the meat doesn't clump together). Cook the beef, in batches, till just brown on the outside. Remove to a sieve to drain.
When ready to serve, heat about 3 tablespoons of the strained used oil in the wok to about 375. Return the beef to the wok, all at once, and stir fry for about 2 minutes. Add remaining sherry, soy sauce and sugar, mixing well. Serve.