Locate your son's prisoner number, which is a required line of information on the address. Use the Bureau of Prisons' Inmate Locator for federal prisoners. Use your state's office for state prisoners. The department is usually called the Department of Corrections and can be found in the phone book.

Find the proper address for your son's prison. Most prison websites have the full address listing. Include your son's entire name, prisoner number, name of the prison and full address when you address the envelope.

Update your son on what is happening with your family or friends to help him maintain ties while incarcerated. Keep it informative, i.e. "Cousin Claire had her baby this week," but take care not to go overboard describing family activities in which your son cannot participate, i.e. a large family trip to Disney World.

Provide encouragement for your son. Reassure him that he is strong enough to serve his time. Encourage him to make the most of prison by participating in educational, religious or recreational activities offered.

Ask your son if he needs or wants anything. Many prisoners can receive credit at the commissary or items such as magazines. Offer these if you are in a position to give them to your son.

Remember that it's OK to tell your son how much you miss him, or even that you're disappointed in him for the incarceration. Be honest and straightforward about how you're feeling, but do not make this the majority of the letter. Doing so can overly frustrate your son and cause him to lose morale in prison.

Include information on when you will visit, if you are able. This gives your son something to look forward to.

Save major news, such as a close relative dying, for a phone call -- if your son is allowed to receive phone calls. A letter is not ideal for such news because your son has no one in the family to react to or grieve with after hearing the information.

Sign off by assuring your son that you love and miss him and will write again soon.