Drink lots of fluids (lubrication) try eating a banana or two (fuel) as well as the warm ups. The water will hydrate you and yor muscles, the banana has a good slow energy release and a good source for playing or low/non aerobic workouts ( which is what playing is, muscles use without a real raise in breathing and heart rate to get more oxygen so more energy for the muscles being used to stop the cramping or getting tired )
Always a good idea to get you instructor to look at your tehnique, the main groups of muscles use use are in the forearm not the hand. If you try to use your hand muscles to use your fingers you will suffer, you have to learn to use the muscles in you forearms.
Main offender in this is the thumb in as much as you make a grip on the neck and in doing so activate the muscle groups to the fingers so they work with the thumb in a pinch, grab or manipulate use so the forces used are distributed among all being used.....you do not need any thumb force to play so do not activate this use.
So try this do not use your thumb on the back of the neck, let it float, now play....feel the difference, your finger movement now comes from deep in the forearms, not the hand. You will have a feeling that the fingers are pulling against the fretboard because the thumb is not there for them to push against. In correct use it is in the forearms you will feel tired or sore not the fingers.
It is this pushing against the thumb rather than the strings is a hidden problem of grip pressure, in other words you squeeze the bass neck, you really just touch the bass neck, you do not grip it with any force. The hand sits there because the body holds it in position against the bass neck, it does not need the bass neck to maintain this position as you can do the same position and movement without a bass ( air guitarist do it all the time LOL)
Now add back the thumb and feel the difference, squeeze the neck and play....feel how you are now pushing against the thumb? relax the hand and the grip, take the thumb off and play...feel the difference now?
Put the thumb back on the bass neck with little or no pressure and feel that the thumb stablises rather then holds, and get that feeling of the the forearms using the fingers to pull the string against the fretboard, rather than the fingers pushing against the thumb.
