It is best to hire a lawyer to put together a parole packet for you. I know that lawyers can be hired for as little as $1500. The parole board could review your husband's papers and decide to set him off for 1 to 3 years without considering his parole again during that time. The parole board looks not only at the offense and surrounding facts but what the person will be returning to - does he have a support system, a job, a place to live, etc.
In Texas, an offender does not actually "go before" the Parole Board in person. It is a paper process, although there are procedures for an attorney and/or family member to make a presentation to the lead voting member - either in person or by telephone conference. You can check the Parole Board Website Publications page for the document "Parole in Texas" for answers to many of your questions. It is not required that an attorney represent an offender being reviewed for parole, but it is often a good idea to at least consult with, if not hire an attorney to help you.
Something else you should consider:
There is an intiative in Texas where prosecutors are putting together parole protest packets now. In fact, the Williamson County DA's office received a grant to travel around Texas teaching other counties how they have successfully lobbied against the granting of parole in many cases. It's their own version of a 'conviction integrity unit'.
If the Parole board hears from the DA's office and not from you, you're not giving them the whole picture on your husband's story. I would strongly suggest following the advice of the previous attorneys on this question and hire a lawyer to assist you.
That being said, let me actually answer your question:
A parole packet should include an opening letter from you on your history with your husband and include all the positive landmarks of your relationship. From when you were married through the birth of any children. Humanize the perception of the board by adding details of his life to the story of your relationship. If he has had a difficult upbringing, try explaning that without using it as an excuse. For instance, "Although he had no father in his life, he still graduated high school and was able to provide leadership as a co-captain of the football team." Turn negatives in to obstacles that he has overcome to show a positive outlook on his history.
Include letters from his family and friends that describe his character in terms of the good things he has done in his relationships with those people. If he attended any church or community programs, ask the leaders of those programs to contribute a letter framing their opinions as to why they believe he will be supported by the community and supportive of the community.
Then outline your ability to support the defendant if he is paroled. Explain where he can live, what job opportunities he may have, how the people of your community can help him integrate back into society.
Include pictures throughout his life depicting the fun and wholesome moments before he was incarcerated.