Playing the violin (and to an even greater extent, the viola) is so much easier when your abdominals are in charge of holding you up. You can train your abs in a ridiculously short time if you build the habit of doing it every day. Here is my strategy for doing this:

Begin by tuning into your abs. Then:

1. remain on the floor, knees bent.
2. lace your hands behind your neck/head for support.
3. suck your abs towards the floor. Picture yourself as a hinge...as your abs contract, they cause your upper body to come off of the floor. Do not try to touch your knees, sit all the way up, etc. Only let your upper body lift if you feel that the abs are doing all of the work.
4. leave your legs as loose as you can. Concentrate all of your mental energy on your abs.
5. leave your arms as loose as you can.
6. leave everything else as loose as you can!

To build the habit of doing these every day, do this exercise until it feels like you're working at it. This is your current limit. Now make the number of repetitions something less than that. The idea is to do a number of repetitions that doesn't feel like work, so that you will be more likely to do it the next day.

I've stabilized at about 60 reps every morning, which is plenty for me to maintain the tone that I need. Some mornings I'm just too tired, so I'll do 30, do something else, then do another 30. You will find the routine that works for you, but always do less than your limit.