There's all kinds of ways.. private lessons, internet video lessons, selling your own CD's and songs, writing music for TV/Film, providing guitar parts either in a studio setting or over the net, making loop and sample CD's, selling merchandise associated with your band. Doing workshops and in-store demonstrations either for pay or to attract students. Selling ring tones.
Over the next decade or so, as the internet speeds up and progresses there's going to be even more money to be made from live video lessons or demonstrations, or collaborations with other musicians and providing your services as a studio musician (at $360/hr minimum).
Also, the good thing about the internet and things like web phones etc, is that once you've recorded something like a video lesson, it's always going to be there making money for you. You can take a long vacation and still be making money from prerecorded video lessons, royalties and CD sales.