These stamina exercises can help to build up your muscles so that you can play for longer amounts of time.  They can also be used to practice proper playing posture. 
The exercises are to be done without making any sound on your violin.

Your goal is to be able to hold up the violin for three (3) minutes (the length of a short song),
while maintaining proper posture and without sagging.

Exercise 1

Hold the violin up, without the bow.
     Place fingers 1, 2 and 3 on their respective tapes on the G string.

         Check for proper stance and posture:

          * feet are shoulder-width apart, and left foot one step forward

          * fingertips are on the tapes, and fingers are rounded

          * neck of violin rests on contact points of thumb and index finger

          * thumb is perpendicular to the floor, across from index finger,
                  and only 1/2 to 3/4 inch of thumb is showing above fingerboard

          * wrist is curved slightly toward the scroll

          * left elbow is swung under violin toward the belly button

Maintain this posture for 15 seconds.   Increase the duration by 15 to 30 seconds each day,  until you are able to hold proper posture for  3 minutes  without sagging

Exercise 2

Hold the violin up, with the bow placed near its tip on the G string.

          Check for proper bow hold:

          * middle finger is slightly wrapped around the bow at the first knuckle (not the tip)

          * thumb is bent, and pushing the bow into the first knuckle of the middle finger

          * ring finger is touching the stick next to the middle finger, and more towards its tip

          * pinky is curved and placed on top of the stick

          * index finger touches the bow between the first and second knuckles

          * top of the hand is turned slightly to the left

          * wrist is bent slightly toward the floor and bow stick is turned slightly toward you

And at the same time .....

       Place fingers 1, 2 and 3 on their respective tapes on the G string.

       Check for proper stance and posture for the left hand, fingers and arm
           as noted in Exercise 1.

Maintain these postures for 15 seconds.   Increase the duration by 15 to 30 seconds each day, until you are able to hold proper postures for  3 minutes  without sagging