shì 是 can be omitted in most cases. However, it may not be omitted under any one of the following conditions.

1. when the subject is a demonstrative pronoun such as zhè 这or nà 那;


2. when the sentence is negative;


3. when the emphasis is on a situation which has caused a fact.

1. 
   Wǒ (shì) zuò huǒchē lái de.
   我坐火车来的。
   I came by train. It was by train that I came.
   
   
   shì 是 may not be omitted in a negative sentence:


2. 
   Wǒ bú shì zài Shànghǎi dǎ de diànhuà.
   我不是在上海打的电话。
   It was not in Shanghai that I called. I did not call in Shanghai.
   
   
   shì 是 may not be omitted if the subject is a demonstrative pronoun such as:


3. 
   Zhè shì zài Běijīng mǎi de.
   这是在北京买的。
   This was bought in Beijing. It was in Beijing that this was bought.
   
   
   shì 是 may not be omitted if the emphasis is on a situation which has caused a fact.


4. 
   Tā rén hěn hēi, shì shài de.
   他人很黑，是晒的。
   He is dark and that 's because of his exposure to the sun.