If you are trying to avoid the alcohol, you can sub a little beef broth. If you are looking to add a little flavor, add in a hoisin or plum sauce.
Are you substituting for rice wine? The sherry sounds like a very nice, if not perfect, substitution.
Sorry, misread your question! Rice wine would be a nice substitute, or a splash of whatever wine you have around. Or you could go booze-free, as lorigoldsby suggests!
For me, white vermouth is a good sub for just about anything.
If you're going with alcohol, Marsala wine is really nice. Sounds like the recipe wants a bit of sharpness from the alcohol. If you're trying to avoid alcohol you might try apple cider.
I have a favorite Chinese cookbook that suggests gin as a substitute.