Does anyone know how to loosen up while throwing their jab...
my trainer keeps telling me im too stiff...
maybe it cuz my arm and shoulder muscles are tired..
but id like to know if there are any stretches or exercises..or just any advice anyone can give on loosening up so i can snap my punch good.

any advice is appreciated thanks.

Don't tense, don't hunch and don't clench your fists. When any muscle contracts it pulls on others and sets off a series of muscle clenches which means when you throw the punch you're basically resisting yourself. Esp' with the common guard which is alot like the 'peek-a-boo' which is usually the first guard taught (hands up to about top lip, then you dip your chin in meaning your gloves are about nose level) you want to tense up abit and feel solid and secure but don't.

Imagine a punch being like a whip, the energy flows up and snaps out. It's a loose movement. Imagine it flowing like water rather than being pure energy just being pushed out. Your fist is the tip of the whip, your body is the whip, not your arm. However, remember, when you do throw those punches push that force through, don't just do the movement.

Shadow boxing will help ya. Try lowering your shoulders down and keeping less hunched. Try to snap your jab too.

thank you both...ill remember that today when i am shadow boxing and hitting the bag...
like a whip flow out from the body not just the arm...
dont tense up...
and dont hunch...
i think i do hunch when i throw my jab..

Ive been told the same things on occasion. Try swimming it loosens your whole upper body up and you start to flow more when you throw. it also adds a good amount of endurance.

wish i could help, but i have the same problem. iv heard relaxing the shoulders and breathing right helps