Good things to write in a bereavement message include "My condolences for your loss," "I'm thinking of you" and "My heart is with you." Writing about specific details of the deceased, such as an aspect of his personality, adds a personal touch that may help the recipient.
A bereavement message typically begins with an appropriate greeting, such as "Dear" or "My Dear Friend." The writer chooses whether to address the card to one person or the family of the deceased. The message varies based on the writer and his relationship to the deceased. If he didn't know the deceased well, a short message offering condolences is appropriate. If he was close to the deceased or the card recipient, he should write a longer message.
While commonly-used condolence messages are acceptable, the writer should avoid clichés that may offend the recipient. These include "I know how you feel" and "Call if you need anything." Every person handles grief differently, so the writer probably doesn't know how the recipient feels. If the writer wants to help the recipient, he should include a specific offer in his message and follow up with the recipient to fulfill that offer. Telling the recipient to call puts the responsibility on the recipient during a difficult time.