Stand up!

The preferred posture is to stand upright with your bass strapped.

• Sitting on a soft surface like a bed, bending over the bass to watch your fretting hand is unnatural. If you feel stiff or cramped when you get up, your body is warning you. 
• Some players suppose that the lower bend in the bass' body is designed to rest it on the right leg. But right handed players playing the classical guitar place the bend on the LEFT leg when they play. If you rest the bass on your right leg, the bass will be too far to the right. There will be too big of a difference between playing seated and standing. So always play standing up..

For a relaxed standing position, place your feet at a comfortable distance, the toes pointing outward to improve stability, knees flexed minimally. In this position, you will feel the rest of your body assume correct posture automatically. The abdomen is stretched, the pelvis is turned backwards, the shoulders are dropped, the chest is widened, the neck is stretched and the chin drops a little. The arms dangle away from the sides. Breath control becomes more easy.

This does not mean standing for a long time is healthy. If you have the opportunity to move around a little when playing, take it. Another option is to sit on a stool with a saddle seat at the highest position still allowing you to keep your feet flat on the floor. With some more attention - it won't come automatically this time - you can reach a position similar to standing.

Do not bend over constantly to watch your fretting hand. Make corrections by ear, not visually. Sit or stand up straight, let your shoulders hang down to the rear, elbows away from the body. Keep your chin a bit low and stretch your neck properly. Breathe deeply and quietly.