Quick Condolence says that simply stating how sorry one is for the loss is usually a good place to start. Add further details about how much the person was loved, as appropriate.
Too often, people worry more about how to somehow lessen the loss or cheer up the grieving family and friends. However, it is not possible to lessen the sorrow; and, it should not be attempted. One need not worry about sounding trite or stupid. Sincerity is the true hallmark of a good condolence note; a few simple heartfelt words about personal sorrow for the death and the sorrow others are feeling is usually the best option. Family members appreciate one sharing a sweet memory or two. A familiar quote can also be appropriate.
Things to avoid mentioning in a condolence note are comparisons of pain felt under a similar tragedy. Although at a later date that might be helpful, immediately after a loss this comment can minimize a friend or family members personal grief. Those agonizing over a death in the family typically do not appreciate platitudes of the type that God took the deceased for a reason or that the deceased is in a better place. These phrases are meant to be comforting but instead come across as critical of those who truly mourn the separation. Certain phrases can suggest that a grieving individual is selfish for not being thrilled their loved one is now in a happier place.