Zoltan has instructional videos for both bass guitar and double bass. Since I am struggling to become a double bass player, I concentrate on those videos.
When I first got my double bass, I was impatient, and ignored videos telling me how to use the "hand shake," the manner in which a bass player uses his left hand. As a result, I became tired quickly and couldn't practice too much.
It has since become obvious to me that solidly learning the basics is highly important, so I am currently concentrating on intonation (playing clear sounding notes), shifting the left hand to accurately play notes up and down the neck, and strengthening my fingers through bass exercises. Zoltan explains these things in clear language that the student can understand -- there are many good bass instructors online, but many of them talk over my head. Zoltan doesn't do that.
I recommend Zoltan if you are looking for a good online instructor. I have added Zoltan's link to my sidebar.