Simple condolence messages include "You're in my thoughts," "I'm sorry for your loss" and "Sending you my deepest sympathy." If the sender has a personal memory of the deceased, he should include it in his message.
When writing a condolence message, the sender should write what comes from his heart and avoid trying to make the recipient feel better. The point of a condolence card is supporting the recipient and letting him know that he's loved. A personal memory of the deceased can help the recipient heal by making him think of good times.
The sender should avoid certain phrases, such as "I know how this feels" or "At least she had a long life." Anything that minimizes the recipient's grief can make him feel worse or even offend him. If the sender is worried about unintentionally offending the recipient, he should write a short message.
If possible, an offer to help the recipient is a nice gesture. However, the sender should make a specific offer and follow through on it himself. For example, he could write "I'm going to take care of your yard" and follow up with the recipient to set up a time for this. This method is more helpful than telling the recipient "Let me know if you need anything," which puts the responsibility of asking for help on the recipient.