Sugar seems to be a popular suggestion to tame spicy food. Some articles I read indicate sugar will calm down the taste buds a little bit. Haven't tried it myself though since I am not a fan of spicy food.

twebst mentioned Chinese vinegar. Do you mean the white one or black kind? Either way, I am not sure that's a good idea.

First, not every Chinese dish will go well with vinegar. Adding vinegar may ruin the dish. Secondly, generally speaking, for a Chinese cook, when s/he finishes cooking one dish, it means this dish is "done" with all the flavors. If the guests (or customers) start sprinkling more salt (soy sauce..etc), it makes the cook feel sort of "incompetent", not doing her/his job. Some people will take it as an insult.

ps. I am a Taiwanese and I cook. I am just sharing my cultural "flavors" here.