Familiarize yourself with the "tools" of the trade. Mixing boards vary by manufacturer, but all provide the same basic functions. Learning how channel strips process audio is essential. Understanding the various ways an audio signal can be directed and re-directed through mixers is characteristic of experienced sound technicians. Learn how to connect amplifiers and speakers, then study crossovers and sub-woofer systems. Outboard equipment and how to use it is a continuous learning opportunity.
Learn basic electronics skills. Soldering defective patch cords is a common emergency repair. Understanding both audio signal flow and electrical power distribution is critical, as finding interruptions in both is a daily part of the job. Learn to read circuit diagrams. Knowledge of electrical safety is important for the health of yourself and the musicians you work with.
Understand how computers are used in audio, from recording and signal processing to controlling MIDI effects. While analog mixers are still commonly used, digital mixers will change that, due to flexibility and extra features. Many techniques will remain the same in theory while the means to use them will change.
Maintain a level of fitness. Loading in and loading out are physically strenuous. Heavy lifting is unavoidable. Using ladders, erecting scaffolding and negotiating tight spaces are regular chores, so be knowledgeable about both the equipment and the safety procedures for using it.
Network with musicians and other sound technicians. Volunteer for local concerts to pitch in and observe the various jobs involved. Intern at local recording studios, theaters or television studios--all places that use mixing boards and sound processing equipment.
Continue to add to your "bag of tricks," and learn additional, related skills, like lighting or perhaps instrument set-up and repair. Large concerts often have both front-of-house (FOH) and side-stage monitor mixers and technicians, so knowledge of the differences in the roles makes you more versatile.
Build your own literal "bag of tricks." Include soldering tools, a multimeter, screwdrivers, pliers and wrenches, but consider such extras as specialty cables, adapters and a variety of gaffer's and electrical tape. Include "human repair" items, such as bandages, pain killers, antiseptic cream, antacids and cough drops. Things like this will make you the "go-to" person and put you in demand.