Hello Advanced Class -- Many of you would benefit from taking some private lessons to work on technique individually. But the next-best thing is to spend time with some of the internet's best strings teachers to help you learn new skills or improve the ones you've already started working on. Here are links to some of the video channels you'll find especially helpful for violin and viola technique. (Looking for cello too...)
ProfessorV (Todd Ehle) - YouTube: www.youtube.com/user/professorV Good all-around technique for intermediate to advanced.
Red Desert Violin (Lora Staples) - YouTube: www.youtube.com/channel/UCu1M3iuyi9PJ9hxn-JARNcg A lot of technique for beginners and Suzuki book repertoire, but also good for intermediate.
Nathan Cole (Los Angeles Philharmonic) - YouTube: www.youtube.com/user/natesviolin/
Violin Masterclass (Kurt Sassmanshaus, Univ. of Cincinnati) - www.violinmasterclass.com Excellent site for beginners through late intermediate, all areas of technique.
Violin Lab (Beth Blackerby) - YouTube: www.youtube.com/user/violinlab Also has a website with subscription-only content.
StringPedagogy.com: The teaching website of Prof. Mimi Zweig of Indiana University. More of a teacher's resource, but has student training videos too. Free to signup and use.