九牛一毛；沧海一粟；杯水车薪
a drop in the ocean/bucket
The money we collected for the victims of the Indian Ocean tsunami disaster is really just a drop in the ocean.

入不敷出
live beyond one's means
My father always told me not to live beyond my means.

入乡随俗
when in Rome, do as the Romans do
A lot of people often say "when in Rome, do as the Romans do."

十之八九
nine times out of ten
When my wife comes home from work, nine times out of ten she buys me an evening newspaper.

十拿九稳；稳操胜券
be in the bag
We're sure to win. The match is in the bag.

力不从心；心余力绌；心有余而力不足
the spirit is willing, but the flesh is weak (这两句的上半句 "the spirit is willing" 和下半句 "the flesh is weak" 常被分开单独使用)
When I asked him for help yesterday, he said "my spirit is willing, but my flesh is weak."

人面兽心；口蜜腹剑；笑里藏刀 (披着羊皮的狼)
a wolf in sheep's clothing
Mary trusted Jack until one day she was cheated by him, and realized that
he was a wolf in sheep's clothing.

人不可貌相
appearances are often deceptive 
"Appearances are often deceptive" is a commonly used idiom, meaning things are not always what they seem to be.