Offer to volunteer at a local club to get experience in live sound if you don't have it already. Two of the most common positions for a roadie are front-of-house sound engineer or monitor engineer. See if you can learn some of the skills at the board from an experienced professional at your local club. There are also schools that have programs in audio engineering.
Acquire guitar-playing skills if you don't have them. You don't have to be a good guitarist, but understanding how the instrument works and what the pedals are for and how to tune the instrument can help you become a guitar tech---another common position in the industry.
Learn about stage lighting and how to operate a lighting board. Again, volunteering at a local club or for a local band can be a great way to gain experience with this. Diversifying your skills will help you move up the ladder from lugging around heavy equipment.
Network extensively with others in the industry. Many touring jobs are never posted but instead are offered to a friend of a friend. Be able to talk about some of the skills you have and bands you have worked for or clubs at which you have worked.
Go online to look for roadie positions at industry boards and sites like Craigslist and Roadiejobs. Have a resume ready if needed, and keep it updated with all your experience.