Funerals are difficult, for a number of reasons, for everyone involved, but especially because there are so many things that could be said about the deceased but often not a clear, simple way to say them. Signing a guest book provides this opportunity, whether it is written by hand in a traditional book at the entrance to the service or typed up online. A few guidelines make it easier to express your condolences or share your thoughts with survivors.
Standard condolences are appropriate for nearly all bereavement situations. You can write, "Our thoughts and prayers are with you and your family" or "She was loved by many" or "He made every moment count." These condolences should be upbeat and positive, giving comfort and support to surviving family members who will later look through the guest book from the funeral and see what people wrote. These notes should be short and to the point and work for both a handwritten and online guest book.
When more space is available, as in an online book, a personal memory of the deceased could be shared, a story about his life that only this guest remembers, or a special anecdote. This gives a diverse cross-section of the impressions the deceased made on the guests' lives and records a number of memories of the deceased that the surviving family members would not have had otherwise.
These can be about the nature of life or death, or something that sums up the deceased's personality. Quotes like "Life is precious" or "Get busy living or get busy dying" are possibly appropriate, depending on the deceased's personality and his relationship with the signer. Borrowing a poem or a statement by a world leader or dignitary may work well.